Classification of Gemstone  l  Identification of Gemstone  l  Colour of Gemstone  l   Absorption of Light  l  
Microscopic Analysis  l  Specific Gravity  l  Refractive Index  l  Thermal Conductivity  l  Enhancement of Gemstone

Classification of Gemstone

All gemstones can be classified as either:

1.    Amorphous

2.    Crystalline

Amorphous

i.No orderly internal atomic structure.

     ii.        No naturally-occurring characteristic shape.

    iii.        Products of rapid cooling.

    iv.        Physical properties constant in all directions.

Examples:

• Jet
• Opal

Crystalline

Crystalline gemstones are characterized by:

      i.        Definite & regular internal atomic structure.

     ii.        Geometrical external forms.

    iii.        Directional properties.

    iv.        Products of slow cooling.

     v.        Identical in all crystals of a given species.

All crystalline gemstones can be classified into the following crystal systems:

1.    Cubic

2.    Tetragonal

3.    Hexagonal

4.    Trigonal

5.    Orthorhombic

6.    Monoclinic

7.    Triclinic

Identification of Gemstone

The identification of gemstones relies on their unique physical, chemical and optical properties. These include:

Specific Gravity

The weight of a gemstone in air compared to the weight of an equal volume of pure water at 4 degrees Celsius.

Example: If a gemstone weighs 8 carats in air and 6 carats in water the "Specific Gravity" is said to be 4.00 or 4 times its volume.

1 cc at 4 degrees Celsius = 1 gram.

Cleavage

The tendency of a crystalline substance to split parallel to certain definite directions (when force is applied) producing more or less smooth surfaces. It is strictly a directional property and can only occur in crystalline substances. It is due to weaknesses in the orderly placement of the atoms within a crystal. Similar to a grain of wood.

Examples:

• Diamond  

• Topaz

• Calcite

Hardness

The ability to resist scratching or abrasion when a pointed fragment from another Mineral is drawn across its surface with insufficient force to cause cleavage. The Moh’s Hardness scale is commonly used to measure the hardness of gemstones using ten minerals with a pre-determined hardness. Gemstones with higher numbers have the ability to scratch gemstones with a lower number.

Moh’s Hardness Scale

1.    Talc

2.    Gypsum

3.    Calcite

4.    Fluorspar

5.    Apatite

6.    Feldspar

7.    Quartz

8.    Topaz

9.    Corundum

10.  Diamond (140 to 1,000 times harder than corundum)

Toughness

Resistance to crushing or breakage (Jadeite)

Fracture

Describes a break or chip other than cleavage on the surface Occurs in all materials in any directions

1.    Conchoidal - shell-like, concentric rings (Glass)

2.    Splintery - long splintery fibers

3.    Uneven - broken or uneven surfaces

4.    Even - producing flat surfaces that are not noticeably irregular

Refraction

The bending of light when it passes from a rarer medium (Air) into a denser medium (Gemstone)

Single Refraction (Isotropic)

Light passing through a substance is bent from its original path but emerges as a single ray. Only occurs in gem minerals belonging to the "Cubic" crystal system and "Amorphous" material.

Double Refraction (Anisotropic or Birefringent)

Light passing through a substance is split into two rays, which travel at different velocities causing differing amounts of refraction. Occurs in gem minerals belonging to all other crystal systems.

Example: The doubling of the back facets as seen in Zircon.

Transparency

The freedom in which light is transmitted through a substance

1.     Transparent: object appears clear and distinct.

2.     Semi-transparent: blurred.

3.     Translucent: light transmitted but no object seen.

4.     Semi-translucent: light only transmitted through the edges.

5.     Opaque: no light allowed passing.

Lustre

The brilliancy of a stone in reflected light, determined by the amount of incident light reflected from its surface (surface reflection)

1.     Metallic - Pyrite.

2.     Adamantine - Diamond.

3.     Vitreous - Quartz, Ruby, Sapphire.

4.     Resinous - Amber, Some Garnets.

5.     Waxy - Turquoise.

6.     Pearly - Moonstone.

7.     Silky - Tigers-Eye Quartz.

Colour of Gemstone

Colour

Colour is defined as the visual sensations produced upon the retina by light waves of different lengths.

Light

Light is defined as a form of energy which is radiated by means of electromagnetic waves measured in centimetres or nanometres which are equal to one millionth of a millimetre.

Visible Light

White Light

Is composed of an approximately equal mixture of all colours or wavelengths that make up the visible spectrum

Red / Orange / Yellow / Green / Blue / Violet

A Coloured Gemstone in White Light

The Colour we see is the result of the absorption by the stone of various wavelengths of the original white light.

Transparent Stones - absorption occurs as the light passes through the stone.

Opaque Stones - absorption occurs as the light is reflected from the stones surface.

Selective Absorption: The suppression of certain wavelengths or colours in white light. It is caused either by impurities present in the gemstone (i.e. Chromium in Ruby or Iron in Amethyst) or by chemicals in the stones composition (i.e. Copper in Malachite or Manganese in Rhodonite).

Allochromatic Gemstones: Gemstones whose colours are caused by impurities.

Idiochromatic Gemstones: Gemstones who owe their colour to their own chemical composition.

Selective absorption of light in both Allochromatic and Idiochromatic gems is caused mainly by the presence of "Transition Elements".

Transition Elements

Metamerism: Colour change effect seen when a stone is moved from one type of lighting to another (i.e. Alexandrite).

In Alexandrite, there is a broad absorption band in the yellow part of the spectrum.

Alexandrite appears green by daylight since this light is rich in shorter wavelengths and red in artificial light (not fluorescent lighting) since this light is rich in longer wavelengths.

The tungsten lamp is blue-deficient hence the red colour seen in Alexandrite

Spectroscope

The phenomenon termed "Selective Absorption" can be made visible by using an instrument called a Spectroscope. By using a series of prisms or diffraction grating, it is possible to analyze the light as it passes through a gemstone. The result is called an "Absorption Spectrum" in which the colours or wavelengths absorbed by the gemstone appear as dark bands.

Prism Type Spectroscope

1.     Adjust the slit so that the resolution is pertinent to the spectrum being analyzed.

2.     Immerse the stone in cold water to cool it down.

3.     Use a strong, cool, concentrated light source. Fiber Optic is recommended.

4.     Either direct the light through the gemstone (in the case of a transparent stone) or reflect it from the surface (in the case of opaque stones).

5.     Position the spectroscope in such a way as to receive the transmitted light through the slit.

Uses:

1.     Unpolished stones.

2.     To identify treated stones.

3.     Faceted stones that have a refractive index above the normal range of the refractometer.

4.     Identify some synthetics (i.e. Natural Blue Sapphire from its synthetic counterpart)

Diffraction Grating

1.     Utilizes a diffraction grating to disperse the light into the spectral colours.

2.     The diffraction grating consists of a glass plate onto which a series of fine parallel lines have been photographically printed in the region of 15,000 to 30,000 per inch. It produces a series of diffracted beams which appear as an evenly spaced out spectrum.

3.     Use in a similar fashion to the prism type spectroscope.

Uses:

1.     Unpolished stones.

2.     To identify treated stones.

3.     Faceted stones that have a refractive index above the normal range of the refractometer.

4.     Identify some synthetics (i.e. Natural Blue Sapphire from its synthetic counterpart)

Transition Elements

Absorption of Light

Chelsea Filter

Developed jointly in the 1934 by the Gem Testing Laboratory of the London Chamber of Commerce and the Chelsea College of Science and Technology, the Chelsea Filter was primarily used to distinguish Emeralds from its many stimulants.

The filter consists of a combination of two gelatine filters that transmit only deep red and yellow/green light. This combination was chosen since Emeralds transmit light in the deep red but absorb light in the yellow/green.

The best results are obtained when stones are examined under a strong electric light (not fluorescent light). By holding the filter close to the eye with the stone(s) receiving as much light as possible, the following reactions can be observed:

GREEN STONES

BLUE STONES

Exceptions:

Not all Emeralds will appear pinkish or reddish through the filter.

Blue Sapphires which show a purple colour change when viewed in artificial light usually appear red.

Natural Blue Sapphire appears red.

OTHER USES

The Chelsea Filter can also be used to separate Natural Green Jadeite from colour enhanced Jadeite.

If the stone appears red under the filter this is positive proof that the stone has been treated.

If the stone appears green, further tests should be carried out since some colour enhanced Jadeite remain inert under the filter.

Microscopic Analysis

The best way to examine a gemstone is by using a microscope.

The microscope provides better magnification, illumination and mechanical stability. It consists of a set of eyepieces, viewing tube, objectives, coarse and fine adjustments, stage with built in illumination (i.e. Reflected light/dark field illumination). It is an essential tool for the gemologist especially in the identification of synthetics.

Gemological Applications

1.     Detection of synthetics and imitations.

2.     Study inclusions to assist in determining identity or place of origin.

3.     Detect double refraction (i.e. Zircon or Peridot)

4.     Detect composite or assembled stones.

5.     Diamond or Coloured Stone clarity grading.

6.     Proportion grading for both Diamonds and Coloured Stones.

7.     Becke line method of R.I. determination.

8.     Direct method of R.I. determination when fitted with a calibrated scale.

9.     Study interference figures to determine whether it is uniaxial or biaxial when fitted with a polarizer/analyzer.

Specific Gravity

Definition

The weight of a gemstone in air compared to an equal volume of pure water at 4 degrees Celsius.

1 cc of water at 4 degrees Celsius = One gram

Method Used to Determine Specific Gravity

1.    Displacement Method.

2.    Hydrostatic Method.

3.     Heavy Liquids.

Displacement Method

Suitable only for determining the Specific Gravity of large objects

1.     Weigh the object in air.

2.     Fill a Eureka Can with water at 4 degrees Celsius until the water starts to flow from the spout.

3.     Allow the water to stabilize and the overflow to cease.

4.     Place a graduated vessel (marked in cc's) under the spout.

5.     Carefully lower the object to be tested into the water.

6.     Convert the water displaced (measured in cc's and representing the volume of the object) into grams.

7.     Use the S.G formula.

Hydrostatic Method

Procedure:

1.     Weigh the stone in air using a precision scale.

2.     Adapt a beam balance scale by placing a four legged stand over the left hand pan so that the swing is unaffected.

3.     Place on the stand a beaker of water at 4 degrees Celsius.

4.     Using non-capillary wire, make a cage and attach the wire to the weigh pan hanger so that the cage is immersed in the water and stays submerged during each full swing without touching the sides.

5.     Attach a counterpoise to the other weigh pan, shorter than the other side since it will have experienced weight loss by being submerged in the water.

6.     Weigh the stone in the water.

7.     Use the S.G Formula.

Heavy Liquids

Recommended liquids:

Varying S.G's can be achieved by diluting Methylene Iodide or Bromoform with either Benzyl Benzoate or Monobromonapthalene.

Procedure:

1.     Clean the stone to be tested thoroughly.

2.     Place the stone in one of the Heavy Liquids.

3.     Observe one of the following reactions:

• If the stone floats - the stone has an S.G lower than the liquid.

• If the stone sinks - the stone has an S.G greater than the liquid.

• If the stone becomes freely suspended in the liquid - the S.G of the stone is equal to that of the liquid.

Disadvantages of S.G Determination

1.     The Hydrostatic Method is only good for larger stones; fewer than 3.00 carats errors are evident.

2.     The Hydrostatic Method is time consuming.

3.     Heavy liquids can damage porous stones.

4.     Can only be used for unmounted stones.

5.     Heavy liquids are unpleasant to deal with and in some cases

6.     Can be dangerous.

Advantages

1.     Useful when dealing with unpolished stones or stones of high Refractive Index.

2.     Easy to use (Heavy Liquids).

Refractive Index/Refraction

The refractive index of a gemstone provides the single most important piece of information to a gemologist seeking to identify an unknown stone. It is a constant that is measurable to four significant figures (i.e. 3 decimal points) and can allow gems to be distinguished even when their R.I's differ only very slightly.

Refraction

The bending of light when it passes from a rarer medium (Air) into a denser medium (Gemstone).

Single Refraction (Isotropic)

Light passing through a substance is bent from its original path but emerges as a single ray. Only occurs in gem minerals belonging to the cubic crystal system or amorphous materials.

Double Refraction (Anisotropic)

Light passing through a substance is split into two rays, which travel at different velocities causing differing amounts of refraction. Occurs in gem minerals belonging to all other crystal systems.

Example: Doubling of the back facets as seen in either Zircon or Peridot.

Refractive Index

Example: Diamond

In 1621, W Snell, a professor at Leyden University, discovered the

"Law of Refraction" which states:

1.     When a ray of light passes from one medium into another, there exists a definite ratio between the sines of the angle of incidence (NOI) and the angle of refraction (NOR), which is dependent only on the two media and the wavelength of light.

2.     The incident ray, the normal (at the point of incidence) and the refracted ray are all in the same plane (a perfectly level surface).

Method Used to Determine Refractive Index

Approximation of R.I. by Immersion

When a specimen is immersed in a liquid having a similar R.I, the relief is low (i.e. the edges tend to disappear). To approximate the R.I. of an unknown specimen, immerse the stone in one liquid after another until one is found in which it most completely disappears.

Liquids used:

Caution: Avoid using porous stones in the above liquids (i.e. Opal, Turquoise, Chalcedony, Lapis Lazuli)

Critical Angle Refractometer

The Refractometer is based on the principle of "Total Internal Reflection" which occurs as incident light rays strike at angles greater than the critical angle (when traveling from a denser medium into a rarer medium) and are reflected back into the denser medium.

It is an optical instrument arranged to show the critical angle of total internal reflection as a shadow edge, on a scale calibrated in refractive indices.

Total Internal Reflection

The name applies to the phenomenon which occurs when a ray of light traveling through a denser medium to a rarer medium at an angle greater than the critical angle suffers complete reflection back through a denser medium.

Critical Angle of Total Reflection

That angle where a ray of light, traveling from a denser medium to one less dense, is refracted at an angle of 90 degrees to the normal, that is it skims along the surface separating the two media. Any further increases of the light ray angle would cause the refracted ray to turn back into the first medium where it obeys the ordinary "Laws of Reflection".

Disadvantages: 

1.     Cannot measure the R.I. of an unpolished stone or rough.

2.     The top end of the Refractometer is limited by the R.I. of the Refractometer glass prism and the contact liquid.

3.     The highest reading is attainable using high lead oxide content glass which is soft and susceptible to scratching.

Procedure:  

1.     Place a droplet of the contact liquid on the glass prism.

2.     Carefully lower the table of the gemstone onto the liquid and gently press down to ensure optical contact.

3.     Switch on the light source and look for the shadow edge on the calibrated scale.

Distant "Vision" for Cabochons  

1.     Apply the smallest droplet of R.I. liquid onto the glass prism. If the drop is too large, most of it disappears beyond the view of the refractometer.

2.     Rest the cabochon upside down on the spot.

3.     View 12 to 18 inches away.

4.     Locate the spot in the eyepiece.

5.     Move your head up and down until half of the spot is dark and half is light.

6.     When the spot is all light, the R.I. of the stone is lower.

7.     When the spot is all dark, the R.I. of the stone is higher.

Determining Birefringence

There are a number of ways of determining whether a gemstone is doubly refractive.

1.    The Refractometer

2.    The Polariscope

The Refractometer

Doubly refractive stones will display two shadow edges when viewed through the eyepiece of the refractometer. By turning the stone carefully on the glass prism, maximum and minimum birefringence can be calculated by subtracting the lower shadow edge from the higher one. This can be a valuable piece of information to a gemologist seeking to identify an unknown gemstone.

Optical Character

Anisotropic gemstones possess either one (uniaxial) or two (biaxial) directions along which light is not doubly refracted. These directions of single refraction are called "Optic axes".

Both amorphous and crystalline substances can be grouped under these three headings:

Isotropic : Cubic or amorphous.

Uniaxial : Tetragonal, hexagonal and Trigonal.

Biaxial : Orthorhombic, monoclinic and triclinic.

This provides yet another valuable piece of information to the gemologist.

Uniaxial: Show a fixed refractive index for the ordinary ray and a varying one for the extraordinary ray.

Biaxial: The R.I. of either rays or shadow edges varies.

Optical Sign

Uniaxial

Positive: The moving shadow edge has a higher R.I. than the stationary edge.

Negative: The moving shadow edge has a lower R.I. than the stationary edge.

Biaxial

Positive: If the higher edge moves more than halfway towards the lowest shadow edge.

Negative: If the lower edge moves more than halfway towards the highest reading.

Polariscope

It is sometimes sufficient simply to know whether a gem stone is singly or doubly refractive. For this uncomplicated test, the Polariscope comes into its own.

Consists of:

1.     Built-in light source.

2.     A protected polarizing filter over the light source which acts as a platform for the gemstone.

3.     A second polarizing filter through which the stone is viewed.

Procedure:  

1.     Place the stone to be tested on the lower platform.

2.     Switch on the light source.

3.     Rotate the top filter until it is in a "Crossed" position and does not allow light passing through the lower filter to pass through the upper filter.

4.     Rotate the stone 360 degrees.

Reaction:  

1.     If the stone is singly refractive it will remain dark as it is turned 360 degrees.

2.     If the stone is doubly refractive it will transmit light in four distinct positions (at 90 degree intervals)

3.     If a crypto-crystalline material is viewed through the filters, it will appear uniformly bright in all positions. This is due to the random orientation of the many minute crystals of which the gemstone is composed.

Caution:

If the stone is viewed along an "Optic" axis (a direction of single refraction) it will appear dark as it is turned. Some stones show "Anomalous Birefringence" caused by internal strain within the stone.

Examples: Spinel, Glass, Diamond.

Thermal conductivity

The Diamond Tester

Diamond has a much higher thermal conductivity than any of its stimulants, and the tester uses this property to identify both mounted and unmounted stones. The instrument is particularly suited to testing small stones that are recessed in their mount.

Consists of:

1.     Hand-held probe that is fitted with a miniature heating sensor.

2.     A control box containing the associated electronic sampling and indicating circuits.

Procedure:

1.     Short pulses of heat are fed to the probe's silver test tip.

2.     The electronic circuits measure the rate at which this is conducted away into the gemstone under test.

3.     A high rate of heat transference will cause the green light to flash indicating that the stone being tested is a diamond.

4.     A low rate of heat transference will cause the red light to flash indicating that the stone is probably not a diamond.

Do's:

1.     Always check the tester against the test plate supplied or a known diamond before using it.

2.     Always allow the rings to cool before testing them if they have just been removed from the hand.

3.     Always test unmounted stones by placing them table down on a test plate and touching the probe to the culet.

4.     Always allow two to three second intervals between tests.

5.     Always check stones twice.

6.     Always return the probe to the holder after use.

7.     Allow all stones to stabilize to room temperature.

Don'ts:  

1.     Do not hold the stone or the mount with your fingers.

2.     Never apply pressure to the probe.

3.     Do not allow the tip of the probe to touch the metal mount.

4.     Rely solely on the results of a diamond tester.

Enhancement of Gemstone

The various enhancements used today are

1.    Heat Treatment.

2.    Irradiation.

3.    Impregnations.

4.    Surface Modifications.

5.     Composite or Assembled Stones.

Heat Treatment

Is defined as the controlled heating of certain stone in order to effect a change of Colour. Certain stones are amenable to such heating and are considered quite stable while others are not. Since no two stones are alike, each possesses the potential to react differently. Stones in which the heat treatment is considered stable are considered commercially acceptable.

Detection

Detection for the most part is difficult. In some cases, identification is based on the fact that certain colours are rarely found in nature and are therefore considered to be enhanced. In other instances, the procedure is so widespread (i.e. Citrine Quartz) that it is automatically assumed to be treated.

Example: Blue Zircon which is produced by heat treating Brown Zircon.

Irradiation

Confined to a smaller group of gemstones, this process can occur in a variety of ways:-

1.     Radium Treatment

2.     Electro-magnetic bombardment

3.     Neutron Radiation

4.     Electron Radiation

Radium Treatment

Caused through exposure to alpha particles emitted by radium salts.

Detection:

Stones are radioactive and will fog a photographic plate if left in contact for several hours in a light proof box. The colouration (green in the case of diamond) is on the surface only and is normally detectable upon close examination.

Electro-Magnetic Bombardment

Bombardment by particles accelerated to enormous speeds in a cyclotron. In diamonds, the colour produced varies from green to black dependent on the length of the treatment.

Detection:

Detected by immersion in a highly refractive liquid since the colour is on the surface only. Yellow diamonds produced by further heating show a characteristic absorption band not seen in natural yellow diamonds. Treated stones will invariably show a ring around the girdle or an umbrella effect around the culet.

Neutron Radiation

Irradiation with Neutrons from an atomic reactor. Colouration is through the entire stone rather than on the surface only.

Detection:

Brown and yellow diamonds produced by this method have a characteristic absorption band at 594nm but green diamonds are undetectable except for a variance in colour compared to the naturally occurring stone.

Electron Radiation

Colouration produced by using an accelerator. Colouration in the case of diamonds ranges from a pale blue to a blue green but is on the surface only. Colour resembles a rare type of diamond known as Type ll b diamonds.

Detection:

Detectable by the fact that treated diamonds are non-conductors of electricity while natural blue diamonds are semi-conductors. Surface colouration is evident when the stones are immersed in a highly refractive liquid.

Most irradiated stones are considered stable but there are more cases of instability than in heat treated stones. Again commercial acceptance hinges on the degree of stability while public disclosure is open for debate.

Impregnations

These can include any of the following:-

1.    Bleaching

2.    Impregnation with colourless or Coloured oils, waxes or plastics.

3.     General dyeing

Detection:

1.     In most cases, the stones are merely used to imitate more expensive stones (i.e. Chalcedony ) and can be identified by variances in physical properties and by microscopic examination.

2.     In the case of dyed green Jadeite, identification is possible by the variances in the absorption spectra and by its reddish or pinkish appearance under the Chelsea Filter.

3.     Some stones may fade under strong light.

4.     Immersion in an R.I liquid may reveal localization of colour. ( Should not be used for porous stones )

5.     Rubbing the material with a cloth or cotton swab previously soaked in acetone.

6.     Colour rubbing off on the thread used for beads.

Reasons for using impregnations

Colourless:

1.     Hide surface cracks.

2.     To stabilize material (i.e. Turquoise) or to protect it from acidic skin oils.

3.     Reduce porosity and improve colour.

4.     Produce a hard coating over a roughly ground surface to give the impression of a polished surface.

5.     Provide protection for softer stones.

Coloured:  

1.     Fill cracks thereby improving colour.

2.     To imitate other stones.

Surface Modifications

Can be through the application of any of the following:- 

1.     Wax

2.     Ink

3.     Paint or Varnish

4.     Foil backs

5.     Mirror backs

6.     Synthetic overgrowth.

Composite Stones

Stones constructed of two or more pieces of material which may or may not be genuine. Composite stones can fall into one of the following categories:- 

1.    True Doublets - consisting of two pieces of genuine stone (i.e. Sapphire/Sapphire)

2.    False Doublets - where the pavilion is normally glass or an inferior stone.

3.     Triplets - consisting of three pieces.

Detection:  

1.     Set in a liquid of similar refractive index. In the case of a Soude Emerald, the colourless Spinel crown and pavilion will disappear in Methylene iodide leaving a disc of green gelatine.

2.     Joining line visible under microscopic examination.

3.     Differences in Lustre.

4.     Bubbles where the two planes meet.

5.     Refractometer tests are useful since most crowns are either quartz, Spinel or glass.

6.     Red "Halo" effect seen around the girdle of an Almandine Garnet doublet ( in colours other than red ) when placed table down on a piece of white paper.