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A GLOSSARY OF SOME GEMMOLOGICAL TERMS
 
 
Beryl   Loupe
Blemish   Mogok
Brilliant   Mong Hsu
Carat   Natural
Chantaburi   Oiling
Clarity   Opaque
Colour   Padparadshca
Corundum   Pailin
Crown   Pavillion
Culet   Ruby
Cut   Sapphire
Diamond   Saturation
Dichroism   Semi Transparent
Doublet   Shapes
Emerald   Specific Gravity
Extinction   Star Ruby
Facets   Star Sapphire
Fancy Sapphire   Surface Diffusion
Girdle   Synthetic
Heat Treatment   Table
Hue   Translucent
Imitation   Transparency
Inclusion   Transparent
Intensity   Trapiche Emerald
Irradiation   Triplet
Kanchanaburi   Windowing
         
 
 
 
 
BERYL:
The mineral type of emerald, chemical analysis Be3Al2(SiO3)6.
 
BLEMISH:
Characteristics or marks confined to or primarily affecting the surface of a gemstone. Mainly abrasions and scratches, chips and pitting.
 
BRILLIANT:
A style of cut designed in the early 20th Century to maximise the beauty of diamonds. It has 58 FACETS. The topmost is the Table (1), below that Bezel (8), then Star (8), then Upper Girdle (16), then Lower Girdle (16), then Pavilion Main (8), and last, right on the bottom, the Culet (1). TOTAL 58 FACETS.
 
CARAT:
A measure of weight for gemstones. The word carat came from the Carob bean which was used as a uniform weight long ago. Now there are five carats to one gram. Percentages of a carat are called Points. One percent is one point.
 
CHANTABURI:
A mining town and gemstone centre in central Thailand near the Cambodian border.
 
CLARITY:
The amount of light which can pass through a gemstone due to a lack of blemishes or inclusions, which make it less valuable. Sometimes the degree of clarity is so marvellous and the freedom from blemishes and inclusions is so complete that value suffers a reverse. We like a blemish to prove it is the real thing.
 
COLOUR:
The HUE of the gemstone, the basic colour, e.g. blue, red or yellow.
 
CORUNDUM:
The mineral type of sapphires and ruby, chemical analysis Al2O3.
 
CROWN:
The top half of the stone above the GIRDLE.
 
CULET:
The lowest point of the stone, where all the FACETS of the PAVILLION come together. It is sometimes a small FACET.
 
CUT:
The work done on the rough gemstone by the cutter, changing it to a faceted or finished stone, in order to make it delightful to the eye, and for it to retain the greatest value.
 
DIAMOND:

Mined in quantity in Africa and Russia. Most stones are cut now in India.

10 on the Moh’s hardness scale (it is the hardest gemstone). Moissanite is the best man-made copy so far.
 
DICHROISM:
Rubies display DICHROISM when two colours are separately displayed when viewed from different directions. An optical property where light is split into two colours which are polarised at right angles to each other. Can be viewed with a Dichroscope.
 
DOUBLET:
An imitation or composite gemstone made of two separate layers glued together, to enhance the stone by either weight or appearance, often sold without proper disclosure.
 
EMERALD:
A gemstone of the mineral type, Beryl. Around 7.5 on the Moh’s hardness scale.
Mined in many countries, most famous are the pure green Colombian emeralds.
 
EXTINCTION:
The black or grey areas seen through the crown, or top, of faceted gems.
 
FACETS:
The flat polished surfaces or planes on a stone.
 
FANCY SAPPHIRE:
Any colour sapphire that is not blue.
 
GIRDLE:
The narrow rim around the stone. It is the largest diameter of the stone.
 
HEAT TREATMENT:
Nearly all rubies and sapphires nowadays are heat treated in order to enhance their colour and make them more valuable. So a pale inexpensive sapphire will be heat treated to around 1700 deg C for a few days to intensify its colour and value. Not recommended to try this at home. It is a special skill.
 
HUE:
The basic colour of a stone, e.g. red, blue or yellow.
 
IMITATION:
A fake gemstone, made not in a lab but by using look-alike materials of a different composition.
 
INCLUSION:
Inclusions are characteristics that are entirely inside a stone or extend into it from the surface. There are many named types of inclusion, which form during the creation of the stone:
Solid crystals may have formed inside the stone: Crystalline Inclusions.
 
Hollow spaces the shape of crystals: Negative crystals or voids.
 
Rutile mineral fibres found in Corundum: Silk.
 
Long needles, solid crystals or tubes filled with liquid or gas: Growth Tubes.
 
Hollow spaces filled with liquid or gas: single phase filled with just fluid: two phase filled with liquid and gas: three phase filled with a liquid, a gas and a solid: this last is common in emeralds: Fluid Inclusions.
 
Cracks, fractures or fissures.
 
Breakage along a weakness: Parting.
 
Flattened planes of the same mineral: Twinning.
 
Circular Fractures surrounding a crystal, sometimes the result of heat treatment: Halo.
 
INTENSITY:
The colour purity of a stone, where other colours such as grey or brown are not present.
 
IRRADIATION:
A treatment of gemstones not generally accepted in the trade as the change of colour in the stone is not permanent. Found in yellow sapphires from Ceylon but not often, as heat treatment is much more common.
 
KANCHANABURI:
A mining town and gemstone centre near the infamous ‘Bridge over the river Kwai’, west of Bangkok.
 
LOUPE:

A jewellers glass, or magnifying glass, the usual power is x 10. A loupe is essential for viewing a stone for any reason.

 
MOGOK:
Region of Burma (Myanmar) where some of the world’s finest rubies have originated.
 
MONG HSU:
Region of Burma (Myanmar) where lesser quality rubies with fissures and suspect fillings and heat treatment have originated.
 
NATURAL:
In the trade, a stone created entirely by nature, but in some instances assisted to its current level of beauty by man. These enhancements should be disclosed by the seller, if asked. For example, an emerald will nearly always be oiled to fill its cracks and make it look good; this is an acceptable enhancement. The type of filling should be disclosed by the seller. In a sapphire, NATURAL often means without heat treatment to enhance the colour. Heat treatment should be disclosed, but it too is an acceptable enhancement.
 
OILING:
Emeralds are nearly all ‘oiled’ professionally. This is an accepted enhancement which fills the natural cracks that emeralds have to make the stone look better. If the oil is coloured it should be disclosed by the seller when asked.
 
OPAQUE:
A measure of CLARITY of a gemstone where no light passes through the stone. All you can see is the surface colour.
 
PADPARADSHCA:
The rarest and most valuable fancy sapphire. A blend of orange and pink pastel hues, light to medium colour, mined in Ceylon (Sri Lanka).
 
PAILIN:
A mining village in Cambodia famous for its excellent medium dark blue sapphires.
 
PAVILLION:
The bottom half of the stone, below the GIRDLE.
 
RUBY:
A RED gemstone of the species Corundum. 9 on the Moh’s hardness scale. Only diamond is harder at 10. Mined in many countries around the world, some of the best stones are thought to come from Burma (Myanmar), Kashmir, Thailand and Ceylon (Sri Lanka).
 
SAPPHIRE:
A gemstone of the species Corundum. 9 on the Moh’s hardness scale. Only Diamond is harder at 10. Medium dark blue is considered the most valuable colour, although sapphires come in almost any other colour, and they are called fancy sapphires.
 
SATURATION:
The colour purity of a stone, where other colours such as grey or brown are not present.
 
SEMI TRANSPARENT:
A state of clarity half way between TRANSPARENT and TRANSLUCENT. It will be cloudy or sleepy.
 
SHAPES:
Traditional gemstones shapes include: Rounds, Ovals, Cabochon, Square Cushion, Rectangular Cushion, Square Emerald Cut, Rectangular Emerald Cut, Square Brilliant Cut (Princess), Trilliant, Rectangular Baguette, Taper Baguette, Pear, Marquise, Heart, Rose, Briolette and Carvings.
There are many modern cuts since the use of lasers and computers, in varying shapes such as flowers, often given a generic name by the designer.
 
SPECIFIC GRAVITY:
Can be used to determine if a gemstone is natural. Specific Gravity is the ratio of a gem’s density to the density of water. The Specific Gravity of Diamond is about 3.52, Corundum is about 4.0, Emerald is about 2.72.
 
STAR RUBY :
A ruby which presents a six rayed star (rarely twelve and never four) when held under a pinpoint light, for example the Sun, or a halogen spotlight. It is cut in a special domed style to allow the light to return off the internal crystal needles, ‘silk’, which forms the star’s rays.
 
STAR SAPPHIRE:
A sapphire which presents a six rayed star (rarely twelve and never four) when held under a pinpoint light, for example the Sun, or a halogen spotlight. It is cut in a special domed style to allow the light to return off the internal crystal needles, ‘silk’, which forms the star’s rays.
 
SURFACE DIFFUSION:
To coat the surface of a cheap colourless gem (often sapphire) with coloured (often blue) crystals and then heat to around 1200 deg C until the crystals have coated the exterior of the gem with a permanent glaze. The stone is then recut and polished. Surface diffused gems are widely used and are often sold openly under a generic name. Detection is possible by an experienced person.
 
SYNTHETIC:
A gemstone which is ‘man-made’ or grown, in a laboratory.
 
TABLE:
The large flat top facet of a stone.
 
TRANSLUCENT:
A measure of clarity where the light that can pass though a gemstone is halfway between perfect TRANSPARENT and OPAQUE. It is difficult to see through this stone.
 
TRANSPARENCY:
A measure of CLARITY. Transparency is the amount of light that can pass through a gemstone, depending on the presence of INCLUSIONS and BLEMISHES.
 
TRANSPARENT:
A high degree of clarity which implies few inclusions or blemishes.
 
TRAPICHE EMERALD:
A cabochon emerald with a black six-rayed star.
 
TRIPLET:
An imitation or composite gemstone made of three separate layers glued together, to enhance the whole by either weight or appearance, often without proper disclosure.
 
WINDOWING:
A poorly cut stone will not reflect all the light back at the observer, and it will be possible to see a ‘window’ through the stone, and maybe read print beneath it.
 
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