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Gem Jargon

There are many terms you may wish to be familiar with as you're purchasing. These are some of the most commonly used and asked about. 

Acid Test
A test to determine the karat fineness on a piece of jewelry.  The test is done by applying acid to the piece, and observing the affect on the alloys in the gold.

Antique Cushion Cut
A gem shape similar to an octagon, but with all curved sides around the girdle.

An individual's opinion as to the value of an item. This value can established for market value, replacement value, or insurance purposes. Usually each successive value class is somewhat higher than the previous listed catagory value.

A single ray, or multiple ray (star) effect seen on some cabochon cut gemstones.

A diamond cut into a small, longish, sometimes tapered four sided shape with parallel faceting running along the pavilion.  Typically used as accent stones.

A gemological family of gems comprised of emerald, aquamarine, morganite, goshenite and golden beryl.

Bezel Set
A method of setting a stone where the entire girdle of a gem is enclosed in metal.

Gem cutting style where the stone is cut with a flat bottom and a domed top.

Gem cutting style where the stone is cut with a flat bottom and a domed top.

Measurement scale most often used to describe the weight of a polished gemstone.  One carat is equal to one fifth of a metric gram.

Method where gold is heated to the melting point, then poured into a mold which has been formed into the shape of the desired jewelry object.  When the gold hardens, it is removed from the mold, polished and, hopefully set with gemstones.

Cat's Eye
A single line reflected ray sometimes seen on a cabochon cut gemstone.

Term used in grading colored gems or diamonds to describe the level of internal inclusions, fractures or stress marks inside of the stone.  Colored stone are assessed with the naked eye, diamonds are graded under 10 power magnification.

A grain property of a gemstone similar to the graining of a piece of wood. Stones can be easily split into along the cleavage line.

Measurement scale used in describing the level of tint in the body color of a diamond. "D" color represents the absence of color, and is the most desired.  Tint levels progress through the alphabet as color levels increase.

The faceted portion of a gem which rises above the girdle.

The bottom end of a gem where the pavilon facets meet.

Critical Angle
The minimum angle at which a stone's pavilion facets can be cut to allow the light entering a gem to be reflected back out of the stone's crown, making the gem brilliant, instead of windowed.  The proper critical angle varies, depending on a gem's refractive index.

The natural form and shape a gems grows into while forming in the ground.

1. Differentiates a faceted and polished stone from an unworked crystal.
2. Used to describe the quality of faceting evenness, diameter to depth proportions, and diameter to table proportions of a polished gem.

The ratio of the weight of a gem to the weight of an identical volume reference material, often water.

The characteristic of some gems to appear to be different colors, or different shades of the same color when the stone is viewed from different sides.

Process where a stone's color is altered through the use of very high heat to melt a coating into the gem's surface.  This coating can be polished off, or worn away over time and use.  Typically used on stones not damaged by high heat - primarily sapphire and ruby.

Diffusion - Deep/Matrix/Lattice
Similar to a standard diffusion process, except in deep diffusion (also called matrix or lattice diffusion) the color altering treatment passes completely through the gem, altering the color of the stone's interior so that it is identical to color on the stone's surface.  This is a permanent treatment process, and far superior to simple diffusion.

Double Refractive
The ability of some gems to split a ray of light into two separate rays, each traveling at a different velocity.  This effect caused the pavilion faceting to appear doubled when the stones is viewed through the table.

A stone which has been constructed by gluing two pieces together. Often used with opal as a top, and onyx as a backer.  Can also be done with colored gems, i.e., ruby on top and garnet on the bottom.

European Gemological Laboratory

Emerald Cut
An octagonal shaped gem cut, possible of near equal width and length, but usually of a longer length than width.  Polished stone has long parallel facets on the pavilion.

A smooth polished face surface on a cut gemstone.

An Inclusion is a stone which has a feather-like appearance.  Often observed in corundum or diamond.

The neon-like glow observed in some gems when they are irradiated with UV light.

Flux Fusion
A high temperature method of growing some varieties of synthetic gemstones.


The outermost edge of a gemstone where the pavilion and crown facets meet.  Usually the area of gem where the mounting assembly secures the gem into a jewelry piece.


Gemological Institute of America. A training facility for gemologists, and a source used to identify or grade various gems.


A term typically used to describe a gem's ability to survive a scratch.  Usually rated on a Mohs' scale.


Heat Treatment
High temperature process used to alter a gem's color.


International Gemological Institute


The bombarding of a stone with gamma rays to alter the color of a gem.


Immersion Test
Test done by dropping a stone into a container of a known density liquid to determine a stones density.  A stone semi-floats in a liquid that is of a density similar to its own.


Broad term used to describe any irregular objects, captured crystals or gasses, fractures or stress marks enclosed in a gemstone.


Jamb Peg
An older method of gem cutting using rudimentary tools.  Usually produces poor results, but can potentially rival a fine modern cut stone when done by a master, expert cutter.


A number used to describe how many of the 24 parts of pure gold are alloy.  Fourteen karat gold has 14 of 24 parts being pure gold, and the remaining 10 parts being alloys.


A flat rotating disk used to cut and polish a stone during the faceting process.


A handheld magnification piece used to view the inclusions in a gem.  10 power is the standard magnification for grading diamonds.


The brilliance of a gem which comes from the light which is reflected off of the stone's surface. (see sheen).


Marquise Cut
A gem cut with its girdle shaped in an elliptical fashion, with both ends pointed (somewhat like an NFL football).


Small, round brilliant cut diamonds, usually under one quarter carat size.


Mohs' Scale
A scale from 1 to 10 which rates a gemstone's hardness to another stone.  Talc, a 1 on the Mohs' scale, scratches when rubbed against a 2 hardness gypsum.  A 3 hardness is scratched by a 4, which in turn is scratched by a 5, and so on. Diamonds are the hardest at a Mohs' 10.  A 7 or better is desirable in jewelry, as it is the hardness of quartz, a primary component of dust.


The mother of pearly secretion of an oyster which forms a pearl.


A naturally occurring fissure or cavity which breaks the surface of a gem.


Old Mine Cut
Early cutting style used on diamonds.  Typically mine cut stones lack symmetry and good proportions.


The iridescent luster of a pearl.


Oval Cut
Stone whose girdle is elliptical in shape.


Name given to a pinkish-orange color of sapphire.


The faceted portion of a gem which falls below the girdle.


Pear Cut
Shaped like a pear fruit, with one rounded end opposite a pointed end.


1. Biwa - Fresh water pearl grown in Lake Biwa, Japan
2. Blister - Thin nacre pearl form which grows against the oyster shell
3. Cultured - Oyster grown, nacre around a foreign substance placed in the oyster by man
4. Natural - Pearly completely formed by nature and an oyster
5. Majorcan - Synthetic pearly made on the island of Majorca


Princess Cut
Square or rectangular shaped girdle, but with faceting somewhat similar to a brilliant cut stone with a pointed culet.


The ratio of a gem's diameter to its table, and depth. Usually expressed as a percentage.


Family of gems comprised of amethyst, citrine, rose quartz, smoky quartz and others.


Radiant Cut
Octagonal, possibly elongated cut gem shape, similar in faceting to a round brilliant cut stone.


The effect of a light-waves being bent as they moves from one medium into another.  Refractive index is a ratio between the speed of light in air, and the speed of light while traveling within a stone.  Highly useful measurement in determining a gemstone's identity.


Round Brilliant Cut
A round shaped gem cut having 58 facets, 33 on the crown and 25 on the pavilion.


The reflected light which come from within a stone. see luster.


A whitish cloud of fine inclusions in a stone which cause a star or eye ray effect to be reflected off a gem's surface.


Tool used to measure the spectrum of light which passes through a gem.  Useful in determining the identity of a gem.


A six ray reflection seen in some cabochon cut gemstones.


Uniformity of a stone's shape and faceting from one side to another.


A man-made gem similar in gemological properties to the naturel stone it imitates.


The largest, crown facet on a polished gem.


The term given to a broad range of processes that a gem may be subjected to in order to improve its apperance.  While most treatment processes are permanent, some may be temporary.


A three layer, glued together composite stone. Opal is often backed (see doublet), and sometimes capped with a clear covering.


UV Light
A type of invisible, high frequency light. Some gems glow when illuminated with UV light.  Sometimes called black light.


Condition where a gemstone is cut below the critical angle (shallow cut, or spread cut), allowing the light to pass through the stone instead of bouncing back and making the gem brilliant.


A striped color banding pattern, often seen in corundum stones.

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