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What is gemology?
Gemology is the branch of science dealing with natural and human made gemstones. It is geosciences hence a branch of mineralogy.
What is a gemstone?
A gemstone is a collective term for all ornamental stones which have the beauty, rarity, durability, portability, tradition and demand.
Gemstones occur from three basic origins.
Inorganic. These are minerals.
Organic obtained from living things.
Human made formed in a laboratory.
What is a mineral?
A mineral is an inorganic crystalline structure with a definite chemical composition, physical properties which are relatively constant.
Gemstones are classified into:
Group Species Variety
Feldspar Beryl
Emerald, Aquamarine
Alexandrite, Chrysoberyl,Cat's Eye Chrysoberyl
Ruby and Sapphire
Garnet Almandine
Blood red
Orange, yellow,
Emerald green
Black Opal, White Opal, Fire Opal
Agates, Cornelian, Onex
Indicolite, Rubellite,Green Tourmaline
All gemstones are further classified as
Amorphous: these gemstones have no orderly atomic structure and no naturally occurring characteristic shape. These are the products of rapid cooling. For example glass, amber, opal etc.
Crystalline: these gemstones have definite and regular atomic structure, directional properties and geometrical external forms. These are products of slow cooling.
Gemstones are basically categorized and identified based on their crystalline structure, specific gravity, refractive index and other optical properties.
Specific Gravity
The weight of a gemstone in air compared to the weight of an equal volume of pure water at 4 degrees Celsius is called its specific gravity.
Formula: Weight in Air

Weight in Air - Weight in Water (Apparent loss of weight in water)
Example: If a gemstone weighs 10 carats in air and 6 carats in water the "Specific Gravity" is said to be 2.50 or 2.5 times its volume.
1 cc at 4 degrees Celsius = 1 gram.
The ability to resist scratching or abrasion when a pointed fragment from another mineral is drawn across gems surface with insufficient force to cause cleavage. The Moh's Hardness scale is 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.  10 no. is hardest than 1no.
Moh's Hardness Scale
Diamond (140 to 1,000 times harder than corundum)
Cleavage is the tendency of a crystalline substance to split parallel when force is applied to certain definite directions 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.
Toughness is the tendency of Resistance to crushing or breakage (Jadeite)
Fracture describes as a break or chip, other than cleavage, on the surface. Occurs in all materials in any directions.
Conchoidal - shell-like, concentric rings (Glass)
Splintery - long splintery fibres
Uneven - broken or uneven surfaces
Even - producing flat surfaces that are not noticeably irregular
Refraction is described as 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)
When light passing through a substance is split into two rays, which travel at different velocities causing differing amounts of refraction is known as double refraction. It occurs in gem minerals belonging to all other crystal system.
Example: The doubling of the back facets as seen in Zircon is the example of D.R.
Transparency is the freedom in which light is transmitted through a substance.
Transparent: object appears clear and distinct.
Semi-transparent: blurred.
Translucent: light transmitted but no object seen.
Semi-translucent: light only transmitted through the edges.
Opaque: no light allowed passing.
Luster is the brilliancy of a stone in reflected light, determined by the amount of incident light reflected from its surface (surface reflection)
Metallic - Pyrite.
Adamantine - Diamond.
Vitreous - Ruby, Sapphire.
Resinous - Amber, Some Garnets.
Waxy - Turquoise.
Pearly - Moonstone.
Silky - Tigers-Eye Quartz.
What Causes Colour in Gemstones?
Colour is defined as the visual sensations produced upon the retina by light waves of different lengths.
Light is defined as a form of energy which is radiated by means of electromagnetic waves measured in centimeters or nanometers which are equal to one millionth of a millimeter.
Visible Light
White Light Is composed of an approximately equal mixture of all colours or wavelengths that make up the visible spectrum (VIBGYOR)
Red / Orange / Yellow / Green / Blue / Indigo / 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 by chemicals in the stones composition (i.e. 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:
Vanadium Synthetic Corundum (Alexandrite Colour change), Blue/Violet Sapphire
Chromium Ruby, Emerald, Alexandrite, Red Spinel, Jadeite,Demantoid Garnet, Pyrope Garnet, Pink Topaz
Iron Amethyst, Sapphire, Peridot, Aquamarine, Tourmaline, Garnet
Nickel Chrysoprase Quartz, Synthetic Green and Yellow Sapphires
Manganese Rhodochrosite,  Spessartite Garnet, Rose Quartz
Copper Malachite, Turquoise, Synthetic Green Sapphire
Cobalt Synthetic Blue Spinel, Blue Synthetic Quartz, Cobalt Glass and Natural Blue Spinel
Titanium Blue Sapphire
Metamerism: Colour change effect seen when a stone is moved from one type of lighting to another (i.e. Alexandrite)
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
Adjust the slit so that the resolution is pertinent to the spectrum being analyzed. Immerse the stone in cold water to cool it down. Use a strong, cool, concentrated light source. Like Fiber Optic. 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). Position the spectroscope in such a way as to receive the transmitted light through the slit.
Unpolished stones.
To identify treated stones.
Faceted stones that have a refractive index above the normal range of the refractometer.
Identify some synthetics (i.e. Natural Blue Sapphire from its synthetic counterpart)
Wavelengths are not linearly spaced out. The red end is bunched whilst the blue/violet is spread out.
Diffraction Grating
Utilizes a diffraction grating to disperse the light into the spectral colours.
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.
Spectrum is not as bright.
Hard to regulate the amount of light that enters the instrument.
Hard to view in the blue end of the spectrum.
Cost, they are relatively inexpensive.
Extremely portable.
Transition Elements:
  • Responsible for attractive green colours.
  • Absorption spectra are similar to chromium but have fewer lines and less distinct broad lines.
  • Responsible for the finest colours in gemstones.
  • Intense and well defined absorption lines.
  • Unabsorbed colours are usually transmitted at almost full intensity.
  • Position, intensity and breadth of yellow/green absorption determine whether the stone is redor green.
  • Colours not as brilliant.
  • Absorption tends to be throughout the entire spectrum but is mostly concentrated in the blue/violet regions.
  • Does not generally appear in gemstones.
  • Overall absorption with no clear cut bands.
  • Reds tend to be absorbed with good transmission in the violet and beyond.
  • Absorption spectra tends to show bands of increasing intensity from the blue down to the violet and beyond.
  • Only appears in idiochromatic gems.
  • Absorption spectra resemble iron but with no clear cut absorption in the visible region.
  • Characterized by distinctive absorption spectra with 3 strong bands in the orange/red, yellow/ green with free transmission in the red.
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 and can allow gems to be distinguished even when their R.I's differ only very slightly.
The bending of light when it passes from a rarer medium (Air) into a denser medium (Gemstone) is called refraction.
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 or Birefringence)
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.
Refractive Index
Formula: R.I. = Velocity of light in air

Velocity of light in a gemstone
R.I. = 186,000 miles per second

72,000 miles per second
= 2.58
Law of Refraction"
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.
The incident ray, the normal (at the point of incidence) and the refracted ray are all in the same plane (a perfectly level surface).
Methods 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:
Water 1.33
Bromoform 1.59
Alcohol 1.36
Iodobenzene 1.62
Petrol 1.45
Monobromonaphthalene 1.66
Benzine 1.50
Methylene Iodide 1.74
Clove Oil 1.54
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 travelling 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 travelling 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, travelling 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".
Cannot measure the R.I. of an unpolished stone or rough.
The top end of the refractometer is limited by the R.I. of the refractometer glass prism and the contact liquid.
The highest reading is attainable using high lead oxide content glass which is soft and susceptible to scratching.
Place a droplet of the contact liquid on the glass prism.
Carefully lower the table of the gemstone onto the liquid and gently press down to ensure optical contact.
Switch on the light source and look for the shadow edge on the calibrated scale.
Distant "Vision" for Cabochons
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.
Rest the cabochon upside down on the spot.
View 12 to 18 inches away. Locate the spot in the eyepiece.
Move your head up and down until half of the spot is dark and half is light.
When the spot is all light, the R.I. of the stone is lower.
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.
The Refractometer
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.
Uniaxial: Show a fixed refractive index for the ordinary ray and a varying one for the extraordinary ray.
Biaxial: The R.I. of both rays and shadow edges vary.
Optical Sign
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.
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.
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.
A polariscope consists of built-in light source. A protected polarizing filter over the light source which acts as a platform for the gemstone. A second polarizing filter through which the stone is viewed.
Place the stone to be tested on the lower platform. Switch on the light source. 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. Rotate the stone 360 degrees.
If the stone is singly refractive it will remain dark as it is turned 360 degrees.
If the stone is doubly refractive it will transmit light in four distinct positions (at 90 degree intervals)
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.
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. For example Spinel, Glass, Diamond etc. 
The Critical Angle
The sine of the critical angle can be calculated using the following formula:
Formula: Sine of critical angle = R.I. of the surrounding medium

R.I. of gemstone
To determine the critical angle of a gem mineral in air:
Formula: Sine of critical angle = 1

R.I. of gemstone
Diamond Sine of critical angle = 1

= .413
The angle itself can be derived from a set of trigonometric tables
Critical angle = Arc sine 0.413 = 24.26 degrees
Quartz Sine of critical angle = 1

= 1.54
Critical angle = Arc sine 0.649 = 40.30 degrees
This means that if a ray of light travelling through a diamond strikes the pavilion facets at an angle greater than 24.26 degrees, it will be reflected back within the stone (Total internal reflection).
If is strikes the pavilion facets at an angle less than 24.26 degrees, it will not be reflected back into the stone.
To achieve "Total Internal Reflection", the lapidary must adjust the angles of the crown and pavilion facets so that the majority of the rays meet the interior faces of the pavilion facets at angles, to the normal, which are greater than the critical angle.
If the angles are wrong, the rays will pass out through the pavilion facets and the stone will appear dark.
It is also important that the rays reflected back from the pavilion facets meet the crown facets at angles less than the critical angle. If they fail to do this, they will undergo "Total Internal Reflection" again instead of being returned to the eye.
When light passes through a doubly refractive gemstone, the light is split into two rays which are polarized at right angles to each other and travel at differing velocities through the gemstone. In some coloured doubly refractive gemstones, these rays may emerge differing in shade or colour. When this occurs, the rays are said to have experienced "Differential Selective Absorption".
Differential Selective Absorption:
Variations in the absorption of certain wavelengths dependent on direction, causing a stone to appear differently coloured in different directions.
D.S.A in:
Singly refractive Gemstones: No D.S.A, stone is the same colour in all directions. Monochroic.
Doubly refractive Gemstones:
Uniaxial - Two colours seen. Dichroic.
Biaxial - Three colours seen. Trichroic
Another term used for Differential Selective Absorption is Pleochroism.
An instrument comprising of a suitably cut rhomb of Iceland Spar (Calcite) and a lens system in a short tube with a small square aperture at the other end.
The dichroscope separates the polarized rays so that they may be observed side by side.
Ace the gemstone on the rotating platform of the dichroscope holder table down.
Focus direct light onto the gemstone.
View the stone through the dichroscope.
Slowly turn the rotating stage so as to view to all directions.
Look for different colours or shades of colour.
Dichroism is only seen in "Doubly Refractive" gemstones.
It is not seen in gemstones that display "Anomalous Birefringence".
May be very weak or even undetectable.
Not seen in colourless doubly refractive gemstones or along an optic axis.
Exact colours are not important.
Helpful in distinguishing:
Blue Sapphire from Synthetic or Natural Blue Spinel.
Ruby from Garnet or Spinel.
Emerald from Demantoid or Tsavorite Garnet.
Any doubly refractive gemstone (i.e. Amethyst) from Glass.
Dichroism can also be used in colour grading coloured gemstones.
I.e. Blue Sapphire described as Greenish/BLUE
Dichroism can also be detected by using a polarizing filter. If it is rotated against a gem, first one colour will be seen (caused by one polarized ray) and then another. It can also be seen simultaneously by using a piece of suitably cut Polaroid.
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 simulates.
The filter consists of a combination of two gelatin 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:
Natural Emeralds Usually appear pinkish or reddish since Emeralds absorb in the yellow/green
Synthetic Emeralds Similar reaction but in most cases they appear a more intense red
Emerald Simulants Pastes (Glass), Soude Emeralds and Doublets appear green
Demantoid Garnet & Green Zircon Similar to Emerald but will produce a negative reading on the refractometer (the scale will appear uniformly dark since no total internal reflection occurs)
Green Tourmaline Green
Chrome Green Tourmaline Red or Pink
Synthetic Green Spinel Green
Peridot & Green Sapphire Green
Synthetic Dark Blue Spinel or Natural Cobalt Blue Spinel Red
Synthetic Light Blue Spinel Orange
Zircon Green
Cobalt Glass Deep red
Aquamarine & Blue Topaz Green
Sapphire Dark green, almost black
Lapis Lazuli Weak brownish-red
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 Spinels appear red.
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 / Darkfield illumination). It is an essential tool for the gemologist especially in the identification of synthetics.
Gemological Applications:
Detection of synthetics and imitations.
Study inclusions to assist in determining identity or place of origin.
Detect double refraction (i.e. Zircon or Peridot)
Detect composite or assembled stones.
Diamond or Coloured Stone clarity grading.
Proportion grading for both Diamonds and Coloured Stones.
Becke line method of R.I. determination.
Direct method of R.I. determination when fitted with a calibrated scale.
Study interference figures to determine whether it is uniaxial or biaxial when fitted with a polarizer/analyzer.
The Diamond Tester
Diamond has a much higher thermal conductivity than any of its simulants, 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:
Hand-held probe that is fitted with a miniature heating sensor.
A control box containing the associated electronic sampling and indicating circuits.
Short pulses of heat are fed to the probe's silver test tip. The electronic circuits measure the rate at which this is conducted away into the gemstone under test. A high rate of heat transference will cause the green light to flash indicating that the stone being tested is a diamond. A low rate of heat transference will cause the red light to flash indicating that the stone is probably not a diamond.
Always check the tester against the test plate supplied or a known diamond before using it.
Always allow the rings to cool before testing them if they have just been removed from the hand.
Always test unmounted stones by placing them table down on a test plate and touching the probe to the culet.
Always allow two to three second intervals between tests.
Always check stones twice.
Always return the probe to the holder after use.
Allow all stones to stabilize to room temperature.
Do not hold the stone or the mount with your fingers.
Never apply pressure to the probe.
Do not allow the tip of the probe to touch the metal mount.
Rely solely on the results of a diamond tester.
Specific Gravity
The weight of a gemstone in air compared to an equal volume of pure water at 4 degrees Celsius is called specific gravity.
Formula: Weight in Air

Weight in Air - Weight in Water
1 cc of water at 4 degrees Celsius = One gram.
Methods for determining Specific Gravity
Displacement Method.
Hydrostatic Method.
Heavy Liquids.
Displacement Method
Suitable only for determining the Specific Gravity of large objects.
Weigh the object in air.
Fill a Eureka Can with water at 4 degrees Celsius until the water starts to flow from the spout..
Allow the water to stabilize and the overflow to cease.
Place a graduated vessel (marked in cc's) under the spout.
Carefully lower the object to be tested into the water.
Convert the water displaced (measured in cc's and representing the volume of the object) into grams.
Use the S.G formula.
Hydrostatic Method
Weigh the stone in air using a precision scale.
Adapt a beam balance scale by placing a four legged stand over the left hand pan so that the swing is unaffected.
Place on the stand a beaker of water at 4 degrees Celsius.
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.
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.
Weigh the stone in the water.
Apply the S.G Formula.
Heavy Liquids
Recommended liquids:
Liquid Specific Gravity
Methylene Iodide 3.32
Bromoform 2.89
Monobromonapthalene 1.49
Benzyl Benzoate 1.11
Water 1.00
Varying S.G's can be achieved by diluting Methylene Iodide or Bromoform with either Benzyl Benzoate or Monobromonapthalene.
Clean the stone to be tested thoroughly.
Place the stone in one of the Heavy Liquids.
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
The Hydrostatic Method is only good for larger stones; fewer than 3.00 carats errors are evident.
The Hydrostatic Method is time consuming.
Heavy liquids can damage porous stones.
Can only be used for unmounted stones.
Heavy liquids are unpleasant to deal with and in some cases
Can be dangerous.
Useful when dealing with unpolished stones or stones of high Refractive Index.
Easy to use (Heavy Liquids).
Gemstone Enhancements
The various enhancements are used:-
Heat Treatment.
Surface Modifications.
Composite or Assembled Stones.
Heat Treatment
Is defined as the controlled heating of certain stones 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 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.
Confined to a smaller group of gemstones, this process can occur in a variety of ways:-
Radium Treatment
Electro-magnetic bombardment
Neutron Radiation
Electron Radiation
Radium Treatment
Caused through exposure to alpha particles emitted by radium salts.
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.
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.
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 llb diamonds.
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.
These can include any of the following:-
Impregnation with colourless or coloured oils, waxes or plastics.
General dyeing
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.
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.
Some stones may fade under strong light.
  Immersion in an R.I liquid may reveal localization of colour. ( Should not be used for porous stones )
  Rubbing the material with a cloth or cotton swab previously soaked in acetone.
  Colour rubbing off on the thread used for beads.
Reasons for using impregnations
Hide surface cracks.
To stabilize material (i.e. Turquoise) or to protect it from acidic skin oils.
Reduce porosity and improve colour.
  Produce a hard coating over a roughly ground surface to give the impression of a polished surface.
  Provide protection for softer stones.
Fill cracks thereby improving colour.
To imitate other stones.
Surface Modifications
Can be through the application of any of the following:-
Paint or Varnish
Foil backs
Mirror backs
Synthetic overgrowth
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