A natural, inorganic substance with a characteristic chemical composition and
Mineral
usually characteristic structure.
Mixed cut A cutting style that combines brilliant-cut and step cut facets.
Abrasions Tiny nicks and pits caused by wear and damage to a gem's facet edges or culet.
, A pattern of dark vertical lines or bands shown by certain gems when viewed
Absorption spectrum
through a spectroscope.
Adularescence The cloudy bluish white light in a moonstone, caused by scattering of light.
Chalcedony with a curved or angular bands or layers that differ in color and
Agate
transparency .
Aggregate A mass of tiny, randomly oriented crystals.
Common name for the Pinctada fucata oyster and the natural or cultured pearls it
Akoya
produces.
Allochromatic A gem colored by trace elements in its crystal structure.
A deposit where gems are eroded from their source rock, then transported away
Alluvial deposit
from their source and further concentrated.
Amorphous Lacking a regular crystal structure.
Aragonite A crystalized form of calcium carbonate found in nacre.
Assembled stone Two or more separate pieces of material joined to form a unit.
Asterism Crossing of chatoyant bands, creating a star in the dome of cabochon.
Atom The basic structural unit of all matter.
A glittery effect caused by light reflecting from small, flat inclusions within a
Aventurescence
gemstone.
Background color An opal's bodycolor, independent of its play-of-color.
A bead used as the core of a cultured pearl, usually made from a freshwater mussel
Bead nucleus
shell.
Bleaching A treatment that uses chemicals to lighten or remove color.
Blemish Characteristic or irregularity confined to the surface of a polished gemstone.
Trade term for a highly prized, vivid blue adularescence displayed by the finest
Blue Sheen
moonstone.
Bodycolor A gemstone's basic color, determined by its selective absorption of light.
Botryoidal A crystal growth habit with a bumpy appearance similar to a bunch of grapes.
Boulder opal Thin layers of precious opal, cut to include matrix.
Boule A cylindrical synthetic crystal produced by the flame-fusion process.
Light that eventually returns to the eye after entering a gem through the crown,
Brilliance
reflecting off its pavilion facets, and exiting back through the crown.
Cutting style with triangular or kite-shaped facets that radiate from the center
Brilliant cut
toward girdle.
Cabochon A smoothly rounded polished gem with a domed top and a flat or curved base.
Calcareous concretion A non-nacreous natural "pearl".
Calibrated sizes Gemstone sizes cut to fit standard mountings.
A gem carving style in which the design often a women's profile, projects slightly
Cameo
from a flat or curved surface.
Cavity An opening that extends into a gem from the surface.
Tiny tension cracks in a moonstone that interfere with adularescence and reduce
Centipedes
value.
A process in which ground powder is heated, sometimes under pressure, to produce
Ceramic process
a fine-grained solid material.
A document that indicates a stone's geographic origin, based on its inclusions and
Certificate of origin
trace element chemistry.
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