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cabochon
stone with flat or slightly concave bottom and domed upper part

calibrated goods
a term used by the gemtrade to denote gemstones cut to standardized shapes and sizes so they can be set in pre-fabricated settings.

carat
abbr. ct, plural cts; unit of weight, 1ct = 0.2g

cat´s eye effect
see chatoyancy

chatoyancy
cat´s eye effect caused by reflection of light at needle-shaped inclusions or growth tubes. only visible in cabochon cuts under a single light source (sunlight, spotlight).

checkerboard cut
cut with no table facet, upper part fully covered with lozenge shaped or square facets

CIBJO
Confédération Internationale de la Bijouterie, Joaillerie, Orfèvrerie, Perles et Pierres or World Jewellery Confederation. founded 1926 as BIBOAH, re-named CIBJO in 1961. the task of this council is to record trade practices and nomenclature and to set trade standards accepted worldwide.

cleavage
the tendency of some gems to split along certain crystal planes

colour change
stones may display different colours under daylight and artificial light; the most prominent colour changing stone is alexandrite, a chrysoberyl variety; the phenomenon also occurs in other gemstones e.g. sapphire

concave cut
also "millenniums cut", with concave grooves in the pavilion (the part of a gemstone that lies beneath the girdle)

crown
the part of a gemstone lying above the girdle

cubic zirconia
"cubic stabilized zirconium oxide", often abbreviated to "CZ", very popular diamond imitation. has nothing in common with natural zircon except for the element of zirconium.

culet
the facet produced by cutting off the tip of the stone to prevent damage, old style, usually not found in modern cuts

cushion
cushion shaped cut

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