Glossary

adularescence
an iridescent sheen, or schiller, seen in moonstone caused by the intergrowth of two different types of feldspar with different refractive indexes.
asterism
a luminous star like effect exhibited in some gemstones like star sapphires, garnets and rubies. it is caused by inclusions of tiny, parallel, rutile needles and may result in four, six or even twelve rayed stars.
aventurescence
the effect caused by small inclusions of minerals like mica, hematite, pyrite, or goethite which cause a gemstone to exhibit a glistening or sparkling effect when rotated or moved.
bezel
the ring or band of metal that surrounds a cabochon or faceted stone.
bezel set
a technique of setting a stone in jewelry. the stone is held in place by first soldering the bezel to the base of the piece. next, the stone is inserted and the metal is compressed tightly around the stone.
cabochon
any unfaceted flat-backed stonestone that has been cut with a rounded convex surface with no facets and a flat base. it may be high domed or almost flat and can be cut in round, oval, square or freeform shapes. this form of cutting is most often used on stones like turquoise, lapis, opals, various forms of quartz and other opaque and semi-opaque stones.
carat
carat (not to be confused with karat) is the standard measure of weight used for gemstones. one carat weighs 0.2 gram (1/5 of a gram or 0.0007 ounce). a hundredth of a carat is called a point.
chatoyancy or chatoyance
literally from French, meaning "cat's eye," a phenomenon seen in some minerals when cut a certain way, tiger's eye being one example. a chatoyant gem exhibits a changeable silky luster as light is reflected within the thin, parallel, fibrous bands, an effect due to the fibrous structure of the material.
common opal
refers to opal without a play of color, also to opal without any degree of translucency, marking or body color โ€” includes milk opal, moss opal, pink opal, yellow opal, hyalite and opaque to semi-transparent opal of various ordinary color.
cryptocrystalline
mineral structure in which crystals are so small they are not distinguishable with a microscope.
dichroic
refers to a gem that appears two different colors or shades when viewed from different directions. It also refers to glass that has been coated with a thin layer of metallic oxide. dichroic coatings transmit certain wavelengths of light while reflecting others, creating an interference effect similar to iridescence. the appearance of a particular piece will be different depending on whether light is reflected or transmitted.
facets
small, flat-cut surfaces that create a sparkling effect on transparent stones.
gemstone
decorative material, usually mineral, prized for some or all the qualities of beauty, durability and rarity -- some would add a fourth โ€” a long history as an accepted gem material (such as lapis which is really a rock, not a mineral).
beauty
beauty is paramount. There are a number of gems which are easily damaged, but are unquestioned gemstones because of their unique beauty and the lack of a suitable substitute.
durability
rarity
the importance of rarity is variable -- quality amethyst was rare and as expensive as rubies and emeralds until the huge finds in Brazil and Uruguay in the late 1800's brought prices down drastically. (On the other hand, extreme rarity often combined with erratic supply reduces the marketability of a gemstone and prices are lower than one would expect.)
a long history
currently unquestioned gemstones as amber, turquoise, carnelian, coral, jasper, pearl and lapis lazuli might not make the cut if they had not been treasured for thousands of years by every culture that made use of them.
Note
There is some disagreement about the terms precious and semiprecious โ€” some feel these terms should be abandoned. For many years diamond, ruby, emerald, sapphire, pearl and opal were the precious gemstones, defined as "of great value or high price", while semiprecious stones were "of less commercial value." However, the value of a number of the other so called semi-precious gemstones may equal or exceed 99% of the value of diamonds and other precious stones when they are top quality. For example, alexandrite and emerald-green imperial jade may bring up to $10,000 per carat. Many others are in the $500 to $2,000 per carat range of commercial diamonds, rubies, sapphires and black opals. Examples are imperial topaz, demantoid and tsavorite garnet, and rare paraiba tourmaline.
gold
a precious metal that does not oxidize or tarnish as most other metals do. Gold is alloyed with other metals like silver, copper or nickel to make it harder. The ratio of gold to other metals is what denotes the karat content. common gold alloys are made by mixing gold, silver, copper, and/or other metals to produce 14K, 18K,and 22K gold as well as colored golds.
gold-filled
a term referring to a material (usually sterling silver) on which a layer of '10Kt or better' gold is mechanically bonded under heat & pressure to one or more surfaces of supporting base metal, then rolled or drawn to a given thickness.gold has been bonded by fusing. The resulting ingot is rolled or drawn to make sheet and wire. gold-filled items are generally considered life-time products. the gold layer will not quickly wear off as it can in electroplated products. in the US, to qualify as gold filled, the metal must be at least 1/20th gold by weight.
inclusions
markings or foreign bodies found within a stone, sometimes used to identify it. inclusions can be solid, liquid, or gas. some inclusions decrease the value of a stone, but some, like rutile forming asterisms in star sapphires and needles in rutilated quartz and tourmalated quartz, are prized.
intaglio
design in which the subject is cut lower than the background (an example would be a 'concave' cameo), a technique that was commonly used for signatory seals which were dipped in wax and used to seal a letter or document.
iridescence
reflection of light off internal features in a gem, giving rise to a rainbow like play of colors.
karat
karat(abbreviated Kt or K), not to be confused with "carat," is the measure of the fineness of gold equal to 1/24 part. pure gold is 24 karat gold, 18K gold is 18/24 gold, 14K gold is 14/24 gold, 12K gold is 12/12 (exactly half gold).
labradorescence
effect which causes dark, metallic-like color shimmers, commonly blue and green, to be displayed on a few minerals, in particular orthoclase feldspars such as labradorite and moonstone.
lariat
a necklace without a clasp, worn looped around the neck with open ends that may be tied into a loose knot, fastened with a ring or a brooch, or tied with a "lariat loop".
matrix
a term typically used for boulder opal -- matrix is the rock in which a gem is found, -- usually ferruginous sandstone or ironstone -- also known as the host or parent rock.
microcrystalline
mineral structure in which crystals are too small to be detected by the naked eye.
minerals
inorganic, naturally occurring materials with a constant chemical composition and regular internal atomic structure.
MOH scale
In 1822, Friedrich Moh, a German mineralogist devised a crude but practical method of comparing hardness or scratch resistance of minerals. It has become universally known as Moh's scale. Moh took ten well known, easily available minerals and arranged them in order of their "scratch hardness" -- if a specimen to be tested can be scratched by a known mineral from the list, it is softer than that mineral -- if it, in turn, will scratch another known mineral, it is harder than that mineral.
hardness mineral/gem standard comparison
10 diamond
9 corundum
8 topaz
7 quartz
6 feldspar good steel knife = 6.5
5 apatite common steel nail = 5, glass = 5.5
4 fluorspar
3 calcite copper penny = 3.5
2 gypsum fingernail = 2.5
1 talc
nacre
the smooth, iridescent substance formed around a grain of sand or other foreign matter in the shells of certain mollusks. over time, these layers build up to form a pearl.
opalescence
the milky blue form of iridescence. the term opalescence is commonly and erroneously used to describe the unique and beautiful phenomenon play of color. contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal.
opaque
refers to a gem that does not allow light to pass through it, that is, it is not transparent or translucent. examples include lapis, turquoise and malachite.
organic gem
gem made by or derived from living organisms -- examples include pearls, coral, paua, and amber.
pearlescence
term used to describe a surface with lustrous cloudy spectral colors like one might see in an oil slick or mother of pearl. synonymous with iridescence.
pegmatite
an igneous rock formed as residual liquids from magma cool, often forming large crystals.
play of color
the phenomena of flashing or moving colors in precious opal due to diffraction and not related to the body color.
pleochroic
term used to describe a gem that appears two or more different colors or shades when viewed from different directions.
potch
common non-precious opal without diffracted colors.
precious gem
see gemstone.
rock
material made up of one or more minerals.
schorl
black tourmaline, the most common variety of tourmaline.
semi-precious gem
see gemstone.
schiller
the form of iridescence in a stone caused by the intergrowth of two different types of feldspar, with different refractive indexes.
shiller
see schiller
sterling silver
sterling silver (SS) is 92.5 percent (925 parts) pure silver and 7.5 percent (75 parts) alloy metal and is the primary material used for silver jewelry.
stone
term used for any gemstone.
translucent
translucent stones allow light to pass through them, but the light is scattered (diffused). some translucent stones include moonstones, opals, iolite and carnelian.
transparent
a term used to describe stones that are clear and transmit light so that objects can be seen through the stone. many fine quality stones such as ruby, citrine, or topaz are transparent. however, due to the depth of color or inclusions, stones may not be as transparent as a glass window, for instance.
treatment
a commonly used method to improve the color or appearance of finished gemstones, including heating, bleaching, dying, diffusing and irradiating. some gemstones would not exist in saleable quantities without treatment. for example, heating greenish brown zoisite to 600 degrees celsius produces the currently popular vibrant violet-blue tanzanite (the gem variety of zoisite).
vermeil
pronounced "ver-may," it is derived from the French word for "veneer", describing sterling silver that has been electroplated with at least 100 millionths of an inch of (usually) 24K gold.

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