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|Adamantine: The luster of a diamond; highest possible luster.
Amorphous: Term applied to gem materials that have no orderly crystal structure; opal is an example.
Apex: Faceting term describing the upper point of a rose cut or briolette cut or any other cut without a table facet.
Baguette Cut: A style of step cut, rectangular in outline, with sharp corners.
Barion Cut: A very brilliant cut originally designed for non-round diamonds by Basil Watermeyer. It refers to gems cut with pointed pavilions like round brilliant stones, creating what Watermeyer referred to as a “light fountain effect.” The cut has subsequently been adapted very effectively to many colored stones. The name combines elements of “Basil” and “Marion,” his wife.
Birefringence: A gemological term referring to the strength of double refraction in gemstones.
Brilliance: Brilliance is the ability of a gem to totally reflect light rays that enter it. High refractive index enhances this property. Improper cutting can reduce a gem’s potential brilliance.
Brilliant Cut: Most often refers to the round brilliant cut, sometimes call the “diamond cut.” Round brilliants can be cut in any gem material. Brilliant facets are always triangular and many non-round stones feature brilliant facets (as opposed to rectangular or square “step cut” facets.)
Briolette: Usually an elongated pear-shaped stone covered with rows of either triangular or rectangular facets. They are used as earrings and pendants.
Cabochon: A stone with a smoothly-rounded top found mostly in non-transparent stones like agate and jasper. It takes its name from the French term meaning “like a bald head” which is an accurate description. Some fine quality gems are cut as cabochons (a.k.a. “cabs”) including jadeite, coral, cat’s-eye and star gems, precious opals, fire agates, etc.
Carat: One fifth of a metric gram. See Metric Carat.
Cleavage: The tendency of certain crystalline minerals like mica to split in definite directions. The split pieces have unusually smooth surfaces. Some gem minerals, including diamond, can be split rather easily along certain planes of weakness.
Color Temperature: A number, expressed in degrees Kelvin, describing the spectral properties of a light source. Lower color temperature implies “warmer” colors with more red and yellow; higher color temperature describes “colder,” more blue, light. Color temperature is important in gem photography.
Critical Angle: The angle in a gem’s pavilion facets beyond which total reflection occurs. This angle varies with gem species and is very important to gem cutters wishing to fashion brilliant stones.
Crown: The portion of a faceted stone or cabochon above the girdle.
Culet: A confusing term in the present day. It originally referred to a tiny facet cut parallel to the table at the sharp point of a gem’s pavilion, now very much out of favor. It also sometimes refers to the “culet angle,” or the angle of the facets on the pavilion that reach to the pointed bottom of a gem.
Dichroism: A double refractive mineral’s ability to display two different colors or color shades when viewed from different directions. (See also “Trichroism.”)
Dispersion: Also called “fire.” Sometimes incorrectly confused with “brilliance” and “scintillation.” Dispersion is the colorful result of a gem material’s ability to differentially refract the various spectral colors of white light somewhat like a prism. The strength of dispersion varies from gem to gem.
Double Refraction: The ability of a transparent gem mineral to break light into two separate rays as it travels through the stone.
Doublet: An assembled stone consisting of two parts glued together, often at the girdle. Opal doublets are common, as are faceted garnet and glass doublets. In opal doublets the thickness and quality of the opal layer, along with size, determines value.
Emerald Cut: Octagonal step-cut shapes that are basically square or rectangular. They are most often made from long, narrow crystals like beryl family gems (emerald, aquamarine, morganite, goshenite, heliodor) or tourmaline. Traditional emerald cuts have truncated corners but are sometimes seen as sharp-cornered rectangles known as baguettes. These shapes may be cut in nearly any material including diamond.
Eye Clean: A way of describing a gem’s lack of visible inclusions when inspected only by the eye, without the assistance of magnification.
Facet: A small polished flat plane on the surface of a gem. It is also used as a verb to describe the process of faceting.
Feather(s): A term used to describe feather-like inclusions in gems. They are usually made of many small cavities. They are also called “veils.”
Fire: A synonym for dispersion.
Flaw(s): Any imperfection in a gem’s clarity. Flaws may be internal or external.
Fracture: Any break in a gemstone, internal or external.
Freeform (Gem): Any stone cut without a standard uniform shape or outline.
Girdle: The portion between the crown and pavilion in a faceted stone, usually a thin edge that defines the outline of the gem. A cabochon’s girdle is the bottom edge that forms its outline.
Gram: A standard of weight in the metric system. One gram equals five carats. There are 1000 grams in a kilogram or “kilo.”
Hardness: The resistance of a gem mineral to scratching and abrading. There are several hardness scales with the one devised by Mohs being best known. See also Toughness.
Heat Treatment: Any gem treatment involving heat, usually to improve color or clarity.
Inclusion(s): The general term used to describe foreign materials or cracks, etc. inside gemstones.
Index of Refraction: See Refractive Index.
Luster: The character of light reflected from a gem’s surface. A gem’s natural luster, used in its identification, may be greatly improved by polishing.
Marquise Cut: Gems cut in the marquise shape are pointed at two ends with sides that are portions of a circle.
Metric Carat: One-fifth of a metric gram; a unit of weight used for gems.
Millimeter: Gems are measured in millimeters. One millimeter (mm.) is one-thousandth of a meter. A meter is approximately 39 inches. One inch contains about 25 millimeters.
Mineral: A natural inorganic substance. Its chemistry, crystal structure (if any) and physical and optical properties are consistent.
Mixed Cut(s): A mixed-cut gem cut features both brilliant facets and step-cut facets (see definitions).
Mohs Scale of Hardness: A scale from 1 to 10 that measures the resistance of gems to being scratched. It is not always the best measure of a gem’s durability (see “Toughness” and “Cleavage”).
Opaque: Transmitting no light; the opposite of Transparent.
Pavilion: the part of a faceted stone lying beneath the girdle.
Pear Cut (also Pendeloque Cut): Most commonly called a “pear shape.” A teardrop-shaped gem with a table facet.
Pinpoint: A term that usually refers to very tiny inclusions inside gems.
Photochroism: The process of selective color absorption in different kinds of light. Most gems absorb all but one color, which is the color they present to the eye. Photochroic (color change) gems like alexandrite display one color in daylight and another in incandescent light.
Pleochroism: The property of most doubly refractive colored minerals of showing two or more colors when viewed in different directions by transmitted light. Not the same as Photochroism. See also Dichroism and Trichroism.
Point: A unit of gem weight; one-hundredth of a carat. One carat consists of one hundred points.
Polish: The smooth, lustrous surface induced on a gem’s surface in the cutting process. See also Luster.
Portuguese Cut: There are many variations of the Portuguese cut. It originally referred to round stones with 96 facets on the pavilion and 81 facets on the crown for a total of 176. There are variations with fewer facets but still far more than the 58 of a standard round brilliant typical of diamonds. Many of our stones are cut in modified Portuguese style which gives them far greater scintillation than standard round brilliants.
Radiant Cut: Specifically a patented diamond cut designed to “brilliantize” the emerald cut, using brilliant facets instead of step facets. There are many variations for both diamonds and colored stones. All use triangular brilliant facets except for the flat table facet.
Refraction: The bending of a light ray as it passes from a less dense medium (air) into one of greater density (a gemstone). The strength of a gem’s ability to bend light is an indication of its potential brilliance as well as a principal means of gem identification.
Refractive Index: Also R.I. A number assigned to a material’s ability to bend light. Diamond, at 2.42, bends light a lot while quartz, at 1.55, is much lower on the scale and is much less brilliant.
Round Brilliant Cut: This gem cut, originally the Mazarin diamond cut developed in the mid-1700s, has undergone a long evolution. There are currently hundreds of variations of the round brilliant cut but the term usually refers to the modern American Ideal cut designed by Marcel Tolkowsky. It usually features 57 triangular “brilliant” facets and one flat table facet.
Rose Cut: A gem cut featuring a crown only, with no table and no pavilion. It is an antique diamond cut and is sometimes used for very dark-toned colored gems like almandine garnet to allow more transmitted light through the stone.
Rough: A general term for uncut gemstones.
Scintillation: The apparent ability of a gem to create more rays of light than entered it. It is accomplished by placing numerous facets on the gem, as in the Portuguese cut. When the gem is properly cut for brilliance as well the effect can be stunning.
Specific Gravity: The weight of a substance in relation to an equal volume of water. Water is 1.0.
Step Cut(s): The group of facet cuts characterized by having square or rectangular facets on both crown and pavilion. Tourmalines and emeralds are often fashioned as step cuts.
Symmetry: The geometric state of being divisible into two or more parts, each of which is identical to the other.
Table (Facet): The large flat facet on the crown of many gemstones. It is usually oriented parallel to the stone’s girdle.
Total Reflection: In gemology it refers to facet placement that allows light to be reflected inside a gem so it leaves at approximately the same angle it entered.
Toughness: The ability of a gem mineral to resist breakage; not the same as hardness. Diamond is the hardest of all gems but breaks (cleaves) rather easily; jadeite is much softer but is highly resistant to breaking.
Translucent: Description of a gem material that allows some light to be transmitted through it but does not allow total visibility.
Transparent: Sometimes referred to by the trendy term “crystal,” it is simply a gem material that freely transmits light and allows complete visibility through it. There are degrees of transparency.
Trap Cut: An archaic name for the step cut. It comes from the Dutch word for staircase.
Trichroism: The ability of a gem mineral to transmit three different colors.
Trillion Cut: Any triangular cut. First designed in Amsterdam, Holland as a diamond cut, this generally triangular cut now has many variations. The corners of this shape may be pointed or rounded and the body will vary depending on the stone’s characteristics and the cutter’s preference.
Triplet: An assembled stone. It usually consists of two main portions, top and bottom, separated by a thin layer of cement which can be colored to simulate the color of the gemstone the triplet imitates. In the case of opal triplets a very thin layer of precious opal is sandwiched between a black backing and a transparent cap on top.
Veil(s): Featherlike inclusions in transparent gem materials. They usually consist of tiny liquid or gas-filled cavities within the stone. Sometimes called “fingerprint” inclusions.
Zircon: One of the most important gemstones. It occurs naturally in colorless, red, orange, brown, yellow and green varieties.
Zoisite: A very important gem mineral, one variety of which yields Tanzanite after heating at a low temperature.
Zone (of Color): A band or spot of deep color in a gem material. Some gems like amethyst and sapphire are very prone to color-zoning.