1. Introduction to Gem materials
  2. Gem classification by origin
  3. Corundum Group
  4. Beryl Group
  5. Garnet Group
  6. Some other Important Gems
  7. Organic Gems
  8. Gem Identification
  9. Test
  10. Visual observations
  11. The Polariscope
  12. The Refractometer
  13. Hardness Test
  14. Specific Gravity
  15. The Spectroscope
  16. Chelsea Color Filter
  17. UV Lamp
  18. The Microscope
    How to separate a mix lot of red stones
  19. How to separate diamond from its simulants
  20. How to check assembled Gemstones
  21. Gem Identification work-sheet
  22. Final Test

 

What is Gemology?
Gemology is the science of gem materials, and a specialized offshoot of an older branch of geosciences mineralogy. It covers all technical aspects of gemstones and gem materials; their chemical, physical and optical properties and the methods and instruments employed in the identification and grading of gem materials.

What is a Gem?
A precious stone of any kind, cut and polished for ornament.
By definition, it is one that possesses:
           
            1)  Beauty
            2)  Durability
            3)  Rarity

 BEAUTY
‘The material must have inherent attractive qualities’.

To be considered a gem, a material must possess to a large degree this preliminary qualification. Beauty in a gemstone may depend on such qualities as color, transparency, brilliance, luster and dispersion (fire). Some gems such as colored diamond may possess all these qualities, while others may be known primarily for their color as in the case with emerald, opal, or turquoise. Most will show a combination of these factors; for example, blue sapphire is much prized for its fine transparency, brilliance and color.
DURABILITY

‘The material must be able to retain its attractive qualities’.

Cleavage is a directional property that is related to planes of weakness within some gems. These gems may be difficult to cut and may, if struck, fall into two or more pieces quite easily. Some gems are brittle and show this when their facet edges become easily chipped, whilst others are so soft that they become easily scratched. On the other hand, gem materials that have a high hardness factor keep their polish and good looks for a long period of time.
RARITY

‘The material cannot be too readily available; otherwise its value will fall’.

This is a valued attribute of gems. It is natural for man to esteem most highly the things which are hard to obtain, and this can come about if the gemstone is simply unavailable in the marketplace, or genuinely scarce in nature, even if it does not possess qualities above others of similar beauty. A good example of this is taaffeite, which is not especially attractive, but will fetch a very high price because of its rarity
CLASSIFICATION OF GEM MATERIALS BY ORIGIN

 

1)  NATURAL GEM
A Product of Nature. Natural gems are further subdivided into four categories;
(I) Minerals, (II) Rocks, (III) Organic Gems, (IV) Mineraloids

  1. Minerals: (Crystalline Materials)

            Minerals are naturally occurring, inorganic materials with a fairly constant chemical composition and usually possessing a definite crystal structure. Examples are beryl, topaz, and corundum.

Topaz crystal in matrix

 

 

Crystal:A chemically uniform solid with an ordered internal atomic structure and external form bounded by symmetrically arranged plane (flat) faces.

  1. Rocks: (Crystalline Materials)

            Intergrowth of two or more natural minerals. While the intergrowth may not be orderly, the atoms contained within each component mineral retain their orderly arrangement. Examples used as gem materials are lapis lazuli (a mixture of the minerals lazurite and calcite, with spots of pyrite).

 

 

 

  1. Organic Gems: (Non-Crystalline or Amorphous Materials)

Naturally occurring substances derived from plants or animals.
Examples are coral, jet, pearls, and amber.

 

IV)  Mineraloids: (Non-Crystalline or Amorphous Materials)
Amorphous, inorganic substance lacking a crystal structure. They are mineral-like in nature but lack a crystal structure, and thus cannot scientifically be classified as minerals. Two important mineraloids are Opal (a solidified silica gel), and Obsidian (natural glass).

 

Made by man in the laboratory in the form of:
M
(a) Synthetic AN-MADE GEM MATERIALS
gems, which duplicate all of the physical, chemical, and optical properties of their natural counterpart. Examples are synthetic beryl, synthetic corundum.

 

 

 

(b) Artificial gems, which have no natural counterpart. Examples are the diamond simulants: strontium titanate, GGG (Gadolinium gallium garnet), YAG (Yttrium aluminum garnet), cubic zirconia

 

  1. ASSEMBLED GEM

A gem material (natural or otherwise), which is produced by assembling two or more pieces of like or unlike materials along a distinct junction. Examples are opal triplet, garnet / glass doublet.

 

  1. IMITATION GEM

A gem material (natural or otherwise), which has the same appearance as the gem it imitates. Examples are synthetic cubic zirconia imitating diamond, red spinel as imitation ruby.
Some Physical & Optical Properties of Gemstones

Specific Gravity or Relative Density
It is the ratio between the weight of a substance and the weight of an equal volume of pure water at 4ºC at standard atmospheric pressure.

Hardness
It is the resistance of a gem material to scratch.

Cleavage and Fracture
Cleavage and fracture refer to the characteristic manner in which gems will break when an external force or stress is applied. Some minerals have a special way of breaking parallel to the planes of atomic weakness, creating smooth flat surfaces. This break is called cleavage.

Toughness
Resistance of a gem material to breakage.

Some Physical & Optical Properties of Gemstones

Refractive Index
It is the ratio of speed of light in air to the speed of light in a given medium (gemstone).

Pleochroism:
The property of a mineral (in our case a gemstone) to show two or three different colors, or color shades when viewed through two different directions of the crystal.

Major Groups of Gemstones and Some Important Gems

 

Gemstones are classified into different groups and varieties in terms of their chemical, optical, and physical properties; like chemical composition, refractive index, specific gravity, hardness, color, etc.

Some important gemstone groups we will discuss.

Beryl Group

Chemistry                   Beryllium Aluminum Silicates (Be3Al2 [Si6O18])
Hardness                     7.5 – 8
S.G.                             2.6 – 2.9
R.I.                              1.56 – 1.59
Varieties:
Colorless beryl           Goshenite
Green beryl                 Emerald (due to chromium)
Blue beryl                   Aquamarine (Iron)
Pink beryl                   Morganite ( Manganese)
Red beryl                    Bixbite (Manganese)

Occurrence:
Emerald                      Columbia, Brazil, Zambia, Madagascar,                                          Pakistan (Swat), Afghanistan, India.
Aquamarine               Brazil, USA, Russia, Madagascar, Mozambique
Pink Beryl                  Afghanistan, Brazil, USA, Madagascar,
Mozambique,
Red Beryl                    USA

Corundum Group

Chemistry:                  Aluminum Oxide  (Al2O3)
Hardness:                   9
S.G.                             3.99 – 4.00
R.I.                              1.762 – 1.770

Varieties:
Red variety is called Ruby, other than red color variety is known as Sapphire.

Sources:
Ruby :             Burma, Thailand, Sri Lanka, Afghanistan,                                     Tajikistan and Pakistan (Kashmir).
Sapphire:        Australia, Madagascar, Myanmar, Sri Lanka,
Tanzania, Thailand.

 

Garnet Group

Chemistry:      Silicates of Al, Ca, Cr, Fe, Mg, Mn.
                        L3Al2[SiO4]3
Hardness:       7 – 7.5
S.G.                 Depends on variety
R.I.                  Depends on variety

Varieties
Almandine  - (Purplish Red) Iron & Al silicates
Andradite - (Green) Ca & Fe silicates
Grossular - (Yellow, Green) Ca & Al silicates
Uvarovite -  (Green) Ca & Cr silicates
Pyrope – (Red, Orangy Red) Mg & Al silicates

Occurrence:    Australia, Brazil, Madagascar, South Africa,
Tanzania, Zimbabwe, Brazil, USA.

Spodumene Group 

Chemistry       Lithium Aluminum Silicate
                        LiAlSi206

Hardness         6.5 -7
S.G                  3.18
R.I                   1.66 - 1.67

Varieties:        Kunzite---------- Pink , Violet or Purple
                        Hiddenite-------- Green

Sources :         Afghanistan, Brazil, Madagascar, Myanmar,
Pakistan, USA .

Quartz Group

Chemistry                   Silica  (SiO2)
Hardness:                   7
S.G.                             2.65
R.I.                              1.544 – 1.553

Verities:
Citrine (Yellow )
Amethyst  (Purple)
Ametrine (Yellow Purple Mix)
Milky Quartz
Smoky Quartz (Black or Brown)
Rutiliated Quartz
Agates – Cryptocrystalline form
Occurrence:
Worldwide – Brazil, USA, India, Sri Lanka, Australia, China, Russia, South Africa.

Some other Important Gems

Peridot

Chemistry:      Gems variety of Olivine group (Mg and Iron Silicate)
                         [Mg,Fe]2SiO4
Hardness:       6.5 – 7
S.G .                3.34
R.I.                  1.654 – 1.690
Color:             olive green, yellow-green, brownish-green

Source:           Burma (Myanmar), Norway, Pakistan, Sri Lanka,
USA.
Topaz

Chemistry       Aluminum Fluoro Silicate
                        Al2SiO4(F,OH)2

Hardness         8
S.G                  3.56
R.I                   1.61-1.62
Color:             Colorless, brown, orange, pink,
                        pinkish-purple, yellow.

Sources :         Brazil, Italy, Japan, Mexico,
Nigeria, Norway, Sri Lanka, USA
Pakistan (world’s best topaz at Katlang, Mardan)
Spinel

Chemistry:      Magnesium Aluminum Oxide
                        MgAl2O4
Hardness:       8                     
S.G .                3.60
R.I.                  1.718
Color:             Blue, brown, green, orange, pink, red

Source:           Myanmar (Burma), Sri Lanka, Tajikistan, Tanzania,
                        Thailand

 

 

Tourmaline

Chemistry       Complex boro-silicate of Al, Mg, and Fe

Hardness         7 – 7.5
S.G                  3.01 – 3.26 (normally 3.05)
R.I                   1.624 – 1.654

Color:             All colors, black or gray, colorless, bi-colored,
                        tri-colored

Sources :         Afghanistan, Africa, Brazil, Madagascar, Nigeria,
Sri Lanka, Tanzania, USA, Zambia

Alexandrite

 

Chemistry:      Beryllium Aluminum Oxide
                        BeAl2O4
Hardness:       8.5                  
S.G .                3.72 – 3.74
R.I.                  1.74 – 1.75

Color:             Brown, green, greenish-yellow, reddish-purple,
                        yellow

Source:           Brazil, Burma, India, Madagascar, Russia,
                        Tanzania, and Zimbabwe

 

Zircon
Tanzanite

Chemistry:      Hydrous Calcium Aluminum Silicate
                        Ca2Al3(SiO4)(OH)
Hardness:       6 – 6.5            
S.G .                3.35
R.I.                  1.69 – 1.70

Color:             Blue, brown, gray, greenish-brown, purple, violet

Source:           Tanzania

Chemistry:      Zirconium Silicate
                        ZrSiO4
Hardness:       6 – 7.5            
S.G .                3.9 – 4.7
R.I.                  1.78 – 1.99

Color:             Black, blue, brown, green, orange, red, yellow

Source:           Brazil, Cambodia, Canada, France, India, Italy,
Madagascar, Norway, Russia, Thailand,
South Africa, Sri Lanka, USA

Diamond

Chemistry:      Carbon (C)

Hardness:       10
S.G.                 3.52
R.I.                  2.417

Color:             Colorless, yellow, brown, pink, green, blue, red,
                        purple, black.

Sources:          South Africa, India, China, Canada,
Brazil, Namibia.

 

Opal

Chemistry:
Silica (SiO2) with up to 10 percent water contents

Hardness -----------  6
SG          ------------ 2.10
R.I          ------------ 1.45

Varieties:
a. White opal, white body color with play of color
b. Black opal, black with play of color
c. Fire opal, various shades of reddish orange or yellow without play of color
d. Water opal, transparent to sub translucent body with play of color

Sources:
Australia, Brazil, Mexico, USA.
 
The play of color in opal is caused by interference and diffraction of light by similarly sized silica spheres (0.0001 – 0.0005 mm in diameter).

Organic Gems:
Organic Gems are the product of living organisms, plants and animals. Organic Gems include the following Gems:
Amber, Coral, Pearl, Ivory.

Amber:
Amber is of vegetable origin, consisting of a time-hardened (fossilized) resin from certain coniferous trees, which flourished more than 30 million years ago.

Chemistry:        It is mainly Hydrocarbon (C10H16O)
Hardness          2.5
S.G                  1.08
R.I                   1.54

Varieties:          Pale yellow, brown, reddish or opaque
Inclusions:         Plants debris, small insects 
Source:             Europe, Germany, Poland, Russia, Burma, Romania
Coral:

Chemistry:
Calcium Carbonate (CaCO3).
This is extracted from seawater by a small animal called coral polyp. This animal attaches itself to rocks under water and deposits a hard mass of calcium carbonate on its body. When the animal dies, this skeleton remains and the new generation of coral polyps builds upon this structure. This animal lives in branching colonies in relatively quiet and clear ocean water at depths of 70 to 400 m.

Hardness:         3.5 – 4
S.G.                 2.65
R.I                   1.48

Varieties:          White, rose pink, red, golden-yellow, black
Source: Taiwan, China, Burma, USA, West Indies.

Pearl:

Chemistry:
80 percent Calcium Carbonate (CaCO3)
Pearl is found in the pearl oyster. An irritant, whether in the form of a fragment of the outer shell, a grain of sand, or a tiny marine worm intrudes within the valves of the oyster. As the animal cannot expel the foreign body, he has to accommodate it. The presence of the irritant stimulates the secretion of the shell, which then encloses the irritant material with a succession of regular concentric layers turning it into the form of a pearl.  

 

Color:               White, pink
Hardness:         3.5 - 4
S.G.                 2.60 - 2.78

Occurrence:      Persian Gulf, Gulf of Mexico, Australia, USA,
                        Scotland.
IVORY:

Chemistry:
Calcium phosphate plus organic matter
Ivory is a hard, white substance that comes from the tusks (elongated teeth) of animals such as the Elephant and Sperm Whale. Humans have carved ornamental ivory since prehistoric times.

Hardness:       2.5 – 3
S.G       :          1.7 – 1.93
R.I.                  1.53 – 1.54

Varieties:        Material is translucent to opaque
Color:              Cream, white

Sources:          Africa, Kenya, Tanzania,
                        India, Sri Lanka, Burma, Thailand.

GEM IDENTIFICATION

A Gemologist performs a series of observation and test for identification of a Gemstone. Finally through step by step testing only one identity will fit the stone in question. This is the end point of the exercise of identifying an unknown material in terms of its mineralogical group, species and variety.

The detail of each step will be discussed with practical demonstration on gemstones.

 

  1. Visual Observations

Note the following information:

      1. Is the stone colored or colorless?
      2. Is the stone transparent?
      3. Is the stone translucent?
      4. Is the stone opaque?
      5. How clean is the stone?
      6. Can you see any inclusion?
      7. How is the stone shaped and cut? E.g. Round, step cut, etc.
      8. Rough estimate of weight of stone
      9. Does the stone show any phenomenon like cat’s eye, asterism, color change, etc.

 
The Instruments
Tweezers, 10x Loupe
           
Tweezers:
With the aid of tweezers, the stone can easily be operated to assess color, transparency, and tone. Tweezers have narrow pointed tips, which are grooved inside to obtain a better grip on the stone. Too much finger pressure may cause the stone to fly out of the tweezers. The grip must be firm, but also gentle.

 

10x Loupe:

After noting visual observations, a more detailed examination of the stone may be desired, and it is then necessary to use some sort of magnification.

The standard instrument for carrying out this work is one which will increase the apparent size of the stone by ten (10) times, generally indicated by 10x, as this is sufficiently powerful to reveal most of the internal and external features of a gemstone. Lenses of greater magnification are available (up to 25x), but these lenses have a very critical focus, and limited field of view and working distance, (the distance between the lens and the gem will be much shorter than a 10x loupe), all of which make them more difficult to use.

 

Clarity

Clarity is the relative freedom from any internal defect or irregularity (inclusions) in a gem. In other words, clarity describes the internal cleanliness of the stone. A stone, which is free of inclusions, will have the highest clarity grade. Inclusions are characteristics which are entirely inside a stone or that extend into it from the surface. The GIA (Gemological Institute of America) defines blemishes as “characteristics confined to, or primarily affecting, the surface.”

Generally, clear transparent gemstones with no visible flaws are the most valued. Some gemstone varieties, notably emerald and ruby, typically have some eye-visible flaws so the price structure takes this into account. On the other hand, gemstones such as aquamarine, citrine, and topaz are normally eye-clean, and as a result there is a less tolerance for noticeable flaws for these gemstone varieties

Transparency

When we look at the clarity of a colored gemstone, we also look at its transparency. This describes the degree to which light can pass through a stone. Transparency and clarity are inter-linked; inclusions can block light passing through a stone. With good transparency, objects seen through a stone look clear and distinct.

Gemologists often use the following terms to describe gem transparency:

 

Cutting Shapes and Styles

Broadly, the styles of gem cutting can be divided into faceted gems (those with geometrically shaped flat polished faces) and non-faceted gems (those that do not have geometrically shaped flat polished faces such as cabochons).

Facet: One of the flat polished surfaces cut on a fashioned gemstone.

The cut of a gem is a combination of its shape and faceting style. Shape largely depends on the original shape of the gem rough (i.e., the shape of the raw gem crystal as it comes from the earth). The oval shape is most frequently used, as it best balances beauty and carat weight retention.

 

Various Cutting Styles

Cutting styles are categorized according to the facet shapes and the particular arrangements of these facets. Generally, a faceted gem will fall into one of the cutting styles listed below.

Brilliant Cut

All facets are cut in ‘triangular’ or ‘kite’ shapes. The brilliant cut is the standard for diamond, but also used on other gems.

Step (Emerald) Cut

Next to the brilliant, the other major facet style used is the step cut, also termed ‘emerald cut’. All facets are cut in ‘squares’ or ‘rectangles’ which are arranged in steps. It is used on stones of rectangular, square or angular girdle outline, with corners truncated slightly to prevent chipping or fracture.

 

    

 

The step cut was developed specifically for emeralds to reduce the amount of pressure exerted during cutting and to protect the gemstone from chipping.

Step (Emerald) Cut

Next to the brilliant, the other major facet style used is the step cut, also termed ‘emerald cut’. All facets are cut in ‘squares’ or ‘rectangles’ which are arranged in steps. It is used on stones of rectangular, square or angular girdle outline, with corners truncated slightly to prevent chipping or fracture.

 

Mixed Cut

A cutting style common in Bangkok, where an immense cutting industry exists. The objective of the mixed cut is to retain as much carat weight as possible from the gem rough. The mixed cut, as the name implies, is a mixture of two styles, the brilliant cut and the step cut. 

 

 

 

Mixed cut is most often used for rubies, sapphires and other colored gems, so-called because it combines a brilliant cut crown with a step cut pavilion.

    
Cabochon Cut

A ‘cabochon’ is a polished gemstone with a flat bottom (or slightly rounded bottom) and a convex or rounded domed top. The traditional cabochon is an oval but cabochons can also be fashioned into other shapes including triangles and rectangles.

 

 

 

Cabochons, commonly known as cabs, are the oldest and most common form of gem cutting. Gems cut en cabochon are shaped and polished, rather than faceting.

 

The step cut was developed specifically for emeralds to reduce the amount of pressure exerted during cutting and to protect the gemstone from chipping

 

Weight

The weight of a gemstone can be determined by a weighing balance. Carat is the unit used to express the weight of gemstones, with one carat equal to 200mg, or 1 carat = 0.2 gram.

Brilliance

It is the amount of light returned to the observer’s eye from within the gemstone. Obviously, the more brilliant the gemstone, the higher quality and more valuable the stone will be. Most crystalline gemstones such as Diamonds, Tanzanite, Topaz, Amethyst, Rubies and Sapphires have a wonderful attribute of being brilliant. 

 

Dispersion

It is the breaking up of white light into the spectral colors (violet, blue, green, yellow, orange, and red) by refraction (the bending of light).

 

Luster

Luster is the quantity and quality of light that is reflected from the surface of the stone.

 

Phenomena

Phenomena are described by unusual visual effects shown by some gemstones. Some examples of the stones that display phenomena are: star sapphire, moonstone, and opal.

Chatoyancy: Of all the various types of phenomena, probably the most commonly encountered is chatoyancy or the cat’s eye effect. It consists of single sharp band of light running across the top of a cabochon-cut stone, similar to that of cat’s eye. Chatoyancy results from light reflecting off multitudes of parallel fibrous inclusions or needles within the stone. Some examples of gemstones: chrysoberyl, quartz, and beryl.

 

Asterism: When a gemstone contains multitudes of parallel needle-like inclusions, a cat’s eye effect is created. In some minerals, however, these needle-like inclusions run in more than one direction. Aventurescence: The phenomenon aventurescence is seen as a strong sparkling reflection of light from plates or flakes of another mineral included in a stone. This effect is seen most often in aventurine quartz, sunstone, and the man-made aventurine glass.

Aventurine quartz (quartzite) shows aventurescence due to included plates of fuchsite (green mica). Aventurescence in sunstone is caused by the light reflecting from the hematite platelets included in the stone, and goldstone (man-made aventurine glass) displays this phenomenon due to copper platelets.

 This produces a sort of ‘multiple chatoyancy’, which is termed asterism or the star effect. Examples are star sapphire, star ruby, star diopside.

 

Color Change: Some stones show the change of color phenomenon: if a stone showing this effect is viewed under a fluorescent light source, which is rich in blue-violet rays, it will appear one color, while under incandescent light (rich in orange-red rays), it will appear a different color. The most common gem of this type is alexandrite which looks greenish under fluorescent light and reddish under incandescent light.

Play of Color: Play of color is the phenomenon observed only in precious opal. It is a multi-patchwork of colors found in opals which change when the viewing angle changes. 

In the play of color in opal, the display of color is due to interference and diffraction of light from extremely minute spheres of silica gel (0.0001 – 0.0005 mm in diameter) regularly spaced in a lattice-like arrangement. The spheres are so small that they can only be seen with the aid of an electron microscope.

 

Labradorescence: Labradorescence is found in labradorite which is a dull gray stone. If it is polished, large areas of vibrant colors often will be seen that can change slowly as the gemstone is moved. What causes this is by the obstruction of light from very thin parallel planes that will pass through the gemstone.

 

Adularescence: This effect is found in moonstone as it displays a blue to white sheen (shining effect) flowing in particular directions. The stone is usually cut in a cabochon shape which can make the stone look like a floating, bluish white light. It is seen as the stone is turned and is usually caused by interference (diffusion) of light.

Light Source

The perception of color and phenomenon are influenced by the type of lighting (daylight, fluorescent, incandescent, focused beam, diffused light, etc.) under which one’s observation has been made.

Basically two types of light source exist:

Incandescent or candle light.
Fluorescent or day light.

Incandescent (glowing from heat) lights should not be used for visual observations and color grading of stones because they emit greater amount of the yellow, orange, and red wavelengths. For example, they make rubies look much redder than they really are while blue sapphires seem almost black under incandescent lighting, as incandescent illumination is lower in blue wavelengths.

 

 

 

 

The ‘cool white’ fluorescent lights found in most offices should also be avoided as they emit large amounts of the blue and violet wavelengths which tend to bring out the blue color in gemstones. Again using corundum as an example, this type of lighting would enhance the blue color of sapphires but make rubies look too dark.

What is required is an artificial light source, which reproduces the natural distribution of wavelengths found in daylight. ‘Daylight’ fluorescent bulbs are designed to simulate the balanced light of daylight.

  1. The Polariscope

 

The atoms of minerals are stacked in particular geometrical arrangement. There are seven such arrangements in which the atoms of minerals are stacked, known as ‘Seven Crystal Systems’.

    1. Cubic System              (SR)
    2. Hexagonal System      (DR)
    3. Tetragonal System      (DR)
    4. Trigonal System                      (DR)
    5. Orthorhombic System (DR)
    6. Monoclinic System      (DR)
    7. Triclinic System                       (DR)

Different Gemstones have different crystallization systems. Crystalline gemstones which belong to ‘Cubic’ crystal system are Single Refractive (SR). Gemstones which belong to rest of the six crystal systems are Double Refractive (DR).

Polariscope is the gemological instrument used to check the Optic Character of the stone under examination; whether the stone is SR (single refractive) or DR (double refractive).
Double Refraction in Anisotropic Gems

Certain anisotropic crystals exhibit the optical phenomenon known as "double refraction," where the incident light is split into two separate rays, each with different refractive indices and velocities, and their refractive indices will typically vary between two extreme values.

The mineral calcite has two different refractive indices 1.490 and 1.660 exhibiting strong double refraction. Calcite's block produces two distinct images (double vision) when it is placed over an object or piece of writing.

 

SR Stones                   DR Stones

Diamond                      Sapphire
Garnet                          Emerald
Spinel                           Topaz
Glass                            Kunzite
                                    Peridot
                                    Aquamarine
                                    Tourmaline
                                    Calcite

Practical Demonstration (How to use Polariscope)
Checking SR & DR Stone
Spinel
Aquamarine
Emerald
Garnet
Peridot
Quartz
Glass

The Refractometer

            An optical instrument which works on the principle of Total Internal Reflection and which is used to measure the Refractive indices of various polished Gemstones.

            R.I. Measurement is generally the most useful means of testing . Different Gems have Different R.I Values.
Practical Demonstration:
How  to take R.I. of Emerald, Peridot, Aquamarine, Ruby, Quartz and Glass with the help of Refractometer.

R.I Values of some Gemstones

Beryl                            1.577-1.583
Emerald                        1.577-1.583
Garnet                          1.746
Kunzite             1.660-1.583
Amethyst                      1.544-1.553
Peridot                         1.654-1.690
Diamond                      2.417
Aquamarine                  1.577-1.583
Ruby                            1.762-1.770
Spinel                           1.718
Hardness Test

Hardness is the resistance of a material (gem) to scratch. It is dependent upon the forces holding the atoms of the mineral together. In 1812, a mineralogist, F. Moh devised a scale of hardness into which all minerals can be placed. He selected ten minerals and arranged them in order so that any one mineral could be used to scratch only minerals which are less hard.
Diamond is the hardest material known to mankind.

    1. Talc                  (Least Hard)
    2. Gypsum
    3. Calcite
    4. Fluorite
    5. Apatite
    6. Orthoclase
    7. Quartz
    8. Topaz
    9. Corundum
    10. Diamond                      (Most Hard)

Different Gemstones have different hardness

Aquamarine                  7.5-8
Garnet              7-7.5
Kunzite             6.5-7
Peridot             6.5-7
Spinel               8

In case of rough stone we can use hardness pencils for
checking hardness according to Moh’s scale of hardness.

Practical Demonstration:
           
Checking hardness of Rough Gemstones with hardness
pencils.
Specific Gravity
           
It is the ratio between the weight of a substance and the weight of an equal volume of pure water at 4ºc and at standard atmospheric pressure. The instrument used for this purpose are Hydrostatic Balance and heavy liquids.

Heavy Liquids
When a gem is put into a liquid of relatively lower density then it will sink. If the stone’s density is exactly the same or equal to that of liquid, the stone will remain suspended in the liquid. If the stone is of less density, it will float.
By this method we can guess about the specific gravity of the stone under examination. The liquids’ specific gravities are known to us.
Warning: All these liquids are poisonous and should be handled with care.
More Commonly used Heavy Liquids

Hydrostatic Balance:

How to use it?
Practical demonstration, checking specific gravity values for gemstones like ruby, emerald, peridot, aquamarine, quartz.
Different  gemstone have different  S.Gs.
Emerald                                    2.72
Ruby                                        4.00
Sapphire                                   4.00
Aquamarine                              2.72 + 0.18
Kunzite                         3.18
Peridot                                     3.34
Topaz                                       3.53
Tourmaline                               3.06
Diamond                                  3.52
Quartz                                      2.66
Synthetic Cubic Zirconia           5.65-5.95
6.The Spectroscope:-

           

            The spectroscope is among the most compact of gemological instruments requiring only a portable light source.
            In the spectroscope the wavelengths that have been absorbed by the stone appear as a series of dark lines and or bands along the spectrum. This series  of lines and bands creates a pattern called an absorption spectrum of gemstones and can be a valuable means of identifying the stone.

7.The Dichroscope

 

Light that passes through a doubly refractive (DR) gemstone is split in different directions with varying velocity. The light is absorbed differently in different vibration directions, resulting in color variation known as Pleochroism e.g. iolite, alexandrite.

In minerals with only two rays, two pleochroic colors can be detected, called dichroism; In minerals with three principal vibration directions, three different pleochroic colors can be detected, called trichroism (with only two observed in any one direction).

Ruby and sapphire have two color shades and are pleochroic; in ruby, for example, yellow-red and purplish-red, which distinguishes it from garnet and red spinel, which have no pleochroism.  
PLEOCHROISM IN COMMON GEMSTONES

Andalusite        Yes (trichroism)
Apatite             Yes, weak
Beryl                Yes, but usually weak
Chrysoberyl      Yes, but usually weak
Diopside                       Yes, weak
Garnets                        No
Iolite                 Yes (trichroism)
Opal                 No
Orthoclase        No
Peridot             Yes, but very weak
Quartz              No
Sapphire           Yes
Scapolite          Yes
Spinel               No
Sphene             Yes
Spodumene      Yes (Kunzite, Hiddenite)
Tanzanite          Yes
Tourmaline       Yes
Topaz               Yes
Zircon              Yes
Chelsea Color Filter (CCF)

 

Chelsea color filter is used for quick separation of emerald from its simulants and to  some extent for the separation of natural from synthetic emerald.

The emerald simulants will appear as green under Chelsea Color Filter, while natural emerald will turn pink under Chelsea color filter. In  the case of synthetic emerald, Chelsea color filter will show it as bright red and natural as pink. The Chelsea color filter detects the presence of chromium element.
Fluorescence (UV Lamp)

            Fluorescence Is the emission of visible light by Gemstones that have been exposed to visible or invisible radiations of shorter wavelengths.
The instrument used for this purpose is known as Ultra Violet (UV) lamp.
            Fluorescence data of some of the principal Gem materials
Gemstone                                    Ultraviolet Response
Diamond                                      Blue
Brown Diamond                          Green
Synthetic Cubic Zirconia             Nil
YAG                                             Nil
Strontium Titanate                       Nil
Ruby (Burma)                              Bright red
Ruby (Thai)                                 Dull red
Synthetic Ruby                            Bright red
Emerald                                       Inert
Synthetic Emerald                       Red                                      
Microscope

Gemological microscopes are usually
used to inspect the authenticity of gemstones
by studying the inclusions of gemstones.

Gem inclusions provide the gemologist with
a wealth of knowledge as to its natural or
synthetic origin, its growth environment,
and the conditions under which it developed. The term ‘inclusions’ covers not only foreign substances trapped during growth but also flaws and blemishes in the form of fractures and cleavages. 

Separating Diamond from its Simulants From a mix lot of Stones
           
For example we have a mix lot of different cut stones, Diamond and its Simulants. At first stage we will use Polariscope to separate SR and DR stones. Now we will check the SR stone further to get R.I value if possible. The R.I value of diamond and most of its simulants are simply over the limit of normal refractometer.
Now, the following stones may include:
Diamond, YAG, CZ and Strontium Titanate.

They all have R.I values over the limit of normal refractometer.
Now we will determine S.G  value for each stone. The S.G values are as follows for each stone:
Diamond                      3.52
YAG                            4.55
Strontium Titanate                     5.13
Cubic Zirconia                          5.95
From Specific Gravity values we can separate Diamond from its Simulants, because they have great difference in S.G values.
To further check it, we will use UV lamp, under UV lamp Diamond will
appear as blue.
HOW TO CHECK ASSEMBLED GEMSTONES
Any Gem made up of two or more parts which have cemented or fused together to form a single stone is known as assembled stone.
                        When carrying out identification always suspect an assembled stone. The key features to look for are:

  1. Difference in color, Inclusion between crown and base
  2. Plane of join
  3. Bubbles in joint plane
  4. R.I. Values from Crown and base

            The following is a list of the assembled stones that are still commonly found:

  1. Corundum doublet
  2. Garnet and Glass doublet
  3. Opal triplet
  4. Diamond and diamond simulant doublet

GEMSTONE ENHANCEMENTS

The term enhancement is defined as any treatment or process other than cutting and polishing that improves the appearance (color/clarity/phenomena), durability, value or availability of a gemstone. In today’s gem marketplace, many gemstones have been enhanced by a variety of methods. Such processes may range from simple heating (such as with tanzanite) to high-tech irradiation (such as with blue topaz). 

Gem traders should take enhancement disclosure seriously and guarantee, to the best of their abilities, to provide the customers with complete information on any and all enhancements to which a gem has been subjected. While many enhancements are currently undetectable, e.g. irradiation. 
Some gemstone enhancements are less stable than others, meaning that the appearance of a gem may change with time. 

Types of Enhancements:

Bleaching: The use of heat, light and/or chemicals or other agents to lighten or remove a gemstone's color. This is often accompanied by subsequent dying and/or impregnation. Example: bleached cultured pearl.

Coating: The use of such surface enhancements as enameling, inking, or foiling of films to improve appearance, provide color or add other special effects. Example: coated topaz.

Dyeing: The introduction of coloring matter into a gemstone to give it new color, intensify existing color or improve color uniformity. Example: dyed green jadeite, dyed quartz.

Filling: The filling of surface-breaking cavities or fissures with colorless glass, plastic, or some similar substance. This process will improve durability, appearance and/or add weight. Example: fracture filled ruby.

Heating: The use of heat to alter color, clarity, and/or phenomena. Example: Ruby, sapphire, tanzanite, aquamarine.

Impregnation: The impregnation of a porous gemstone with a colorless agent (usually plastic or wax) to give it durability and improve appearance. Example: Stabilized turquoise, impregnated jadeite.

Laser Drilling: The use of a laser and chemicals to reach and alter inclusions. Example: Laser drilled diamond.

Oiling / Resin Filling: The filling of surface-breaking fissures with colorless oil, wax, resin or other colorless substances, except glass or plastic, to improve the gemstone’s appearance. Example: emerald.

Irradiation: The use of neutrons, gamma, ultraviolet and/or electron bombardment to alter a gemstone’s color. The irradiation may be followed by a heating process.
Example: blue topaz

Lattice (‘bulk’ or ‘surface’) Diffusion: Outside-in diffusion of coloring chemicals via high-temperature heat treatment to produce color and/or asterism. Example: lattice diffusion-treated sapphire.

Sugar Treatment: These simple surface treatments can darken pale opal and enhance its color display. For this purpose the gem is soaked in a hot sugar solution.