Gemstone Glossary

Cabochon

Stone with flat or slightly convex 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

schachbrettschliff checkerboard cutschachbrettschliff checkerboard cut

Chelsea filter

A dichromatic filter which transmits only red and greenish yellow light. Originally developed to distinguish natural emeralds from synthetic ones (early synthetic emeralds glowed strongly red through the filter whereas natural emeralds are inert or show only a moderate reaction). For skilled gemmologists the Chelsea filter provides useful clues (not scientific evidence, mind you!) as to the identity of synthetic blue spinel, irradiated topaz and aquamarine. Deep blue cobalt glass also shows a strong filter reaction.

chelsea filter

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

Tendency of some gems to split along certain crystal planes

Colour change

Stones may display different colours in 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 and garnet.

alexandritesapphire

Concave cut

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

Critical angle

A ray of light, travelling from an optically dense medium, e.g. a gemstone, into a rare medium, e.g. air, is refracted away from the perpendicular. At the critical angle, the ray does not leave the dense medium but travels along the boundary between the two media. Any light, hitting this boundary at an angle greater than the critical angle, cannot leave the dense medium but is totally reflected.

Crown

The part of a gemstone lying above the girdle

Crystal systems

The seven crystal systems and their most prominent representatives are:

  1. cubic: diamond, garnet, spinel, fluorite
  2. tetragonal: zircon, skapolite, rutile
  3. hexagonal: beryl, apatite, jeremejevite, benitoite, sugilite
  4. trigonal: corundum, tourmaline, quartz, chalcite, rhodochrosite, hematite, smithsonite, phenakite, dioptase
  5. orthorhombic: olivine (peridot), chrysoberyl, topaz, cordierite (iolite), zoisite (tanzanite), andalusite, danburite, prehnite, enstatite, kornerupine
  6. monoklin: jadeite, nephrite, spodumen (kunzite, hiddenite), malachite, clinohumite, howlite, brazilianite, charoite, diopside, orthoklas, staurolite
  7. triclinic: turquoise, kyanite, labradorite, mikrokline, oilgoklase, plagioklase, bytwonite, rhodonite, sapphirine

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