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Glossary

Abrasive A material used for grinding or polishing.
adularescence A white or bluish reflection coming from a definite plane in a mineral when the stone is turned. The name comes from the feldspar variety adularia and is best known in the moonstone gemstones.
agate Is a microcrystalline variety of quartz (silica), chiefly chalcedony, characterized by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rock.
Alloy A mixture of two or more metals, or a metal and a non-metal. For example, brass is an alloy of copper and zinc, and steel is an alloy of iron and carbon.
alluvial A gemstone that is created from soil or sediments, deposited by a river, or other running water.
alluvium Clay, silt, sand, gravel, or similar detritus material deposited by running water.
Aluminium A light, ductile and malleable silver-white metal. The chemical symbol for aluminium is Al.
amorphous Amorphous materials are the result rapidly cooling molten material. The cooling reduces the mobility of the material’s molecules before they can pack into a more thermodynamically favorable crystalline state.
appraisal An evaluation performed by a licensed gemologist to determine the replacement value of a gem or piece of jewelry.
Argentite A silver sulphide mineral, the main ore for silver. The chemical formula for argentite is Ag2S.
asterism A star effect displayed by certain gems with intersecting nclusions.
Atomic Mass The mass of one atom of an element.
Azurite A blue mineral containing copper. It is composed of copper carbonate – chemical formula Cu3(CO3)3. It has a hardness of 3.5-4 and produces a blue streak when scratched on a white tile.
baroque Oddly shaped pearls that form during cultivation.
Basalt A dark igneous rock. Basalt originates from lava that has extruded onto the surface of the Earth. Therefore the lava has cooled quickly, forming small crystals.
brilliant cut A round shaped stone that has a minimum of fifty-eight facets.
beryl Beryl is a mineral made up of beryllium aluminum cyclosilicate. Pure beryl is colorless, colored beryl has been tinted by impurities; Green beryl is called emerald, blue beryl is aquamarine. The name comes from the Greek beryllos for the blue-green color of sea water.
cabochon The rounding of a gem without facets into the shape of a highly polished dome.
cameo The art of carving a shell or similar material above its background.
Carat A unit of weight for diamonds, pearls and other gemstones. 1 carat = 200mg. or; A unit used to measure the purity of gold. i.e.

  • 1 carat = 1 out of every 24 parts is gold
  • 2 carat = 2 out of every 24 parts is gold
  • 24 carat gold is pure gold
Cerargyrite Silver chloride; with the chemical formula AgCl, an important silver ore; also called horn silver.
Chalcopyrite A mineral containing copper. Its chemical name is copper iron sulphide – chemical formula CuFeS2. It is brassy yellow in colour, has a hardness of 3.5-4 and produces a green streak when scratched on a white tile.
chatoyancy Derived from the French word, meaning ‘cat’s eye,” chatoyancy is an optical reflectance effect seen in certain gemstones. Chatoyancy is caused by the fibrous structure of a mineral or gemstone such as “tiger eye” quartz.
chatoyant Varying in color when seen in different lights or from different angles.
colloidal silicas Are suspensions of fine amorphous, nonporous, and typically spherical silica particles in a liquid phase.
Compound A combination of two or more different atoms bonded together. For example common table salt is a compound of the elements sodium and chlorine.
conchoidal fracture Describing the way a brittle material separates or breaks when the fracture does not follow any natural plane of separation.
Conduct To transmit or serve as a channel or medium for heat, electricity or sound.
Conductivity The power of conducting heat, electricity or sound. Thermal conductivity is the power to conduct heat; electrical conductivity is the power to conduct electricity. Silver, for example is an excellent thermal and electrical conductor. It transmits both heat and electricity very well.
Copper A fairly soft metal (hardness 2.5-3.0.) It is an excellent conductor of heat and electricity, and is very malleable and ductile. It has a characteristic red colour when fresh (copper-red), but tarnishes to a greenish colour (have a look at an old 1 or 2 cent coin). Copper dissolves easily in acid. Because it is such a good conductor and it is so ductile, copper is mainly used in electrical goods. The chemical symbol for copper is Cu.
Copper Sulphate A compound of copper and sulphur, blue in colour. The chemical formula for copper sulphate is CuSO4.
Corrode To eat away the surface of a solid (especially metal), by chemical action.
cristobalite Cristobalite is a high-temperature polymorph of quartz, meaning that it is composed of the same chemistry, SiO2, but has a different structure.
cryptocrystalline Cryptocrystalline describes a rock’s texture which is so finely crystalline that its crystalline nature is only visible microscopically by viewing a thin section by transmitted polarized light.
crystal Crystal is a solid symmetrical structure in which the atoms and molecules are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions.
Density The density of a substance depends on both its mass and its volume. For example, feathers are less dense than lead. 1 tonne of feathers will have a greater volume than 1 tonne of lead. Similarly, 1 litre of lead will have a much greater mass than 1 litre of feathers.
Density can be calculated by the equation: density%3Dmass (divided by) volume 1 litre of water has a mass of 1 kilogram, therefore the density of water is 1. Gold has a density of 19.3.
dichroism The ability of some gems to display a second color when viewed from a different angle.
dispersion The splitting of light as it enters a gemstone. Also called the stone’s “fire”.
doublet A stone made of two components, generally held together with a clear adhesive.
dop Also known as a “Dop Stick” or “Dop Chuck”, a dop is a rod used to hold the rough gemstone during the grinding process. The rough is usually held to the dop with wax glue.
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