A gemstone is the naturally occurring crystalline form of a mineral, and can be formed either as a single mineral or as a member of a mineral family or group.
To be desirable as gem material, the mineral must be hard, durable, beautiful and valuable in its rarity.
Gem materials are divided into organic and inorganic substances. Organic materials come from living organisms and include pearl, coral, amber, jet and shell.
There are about three thousand known inorganic minerals, but only about one hundred have been cut as gemstones. Although some gemstone varieties have been
treasured since history began, others were discovered more recently.
Many gemstones are coloured, deriving their colour from ‘transition elements’ (colouring elements) within the stone’s chemical makeup, which absorb wavelengths
of white light. These include titanium (blue), vanadium (green), chromium (red and green), manganese (pinkish/pale orange), iron (red, yellow, green and blue),
cobalt (blue), nickel (green) and copper (blue/greenish blue). It is these colours that make the stone so attractive and desirable for jewellery.
Gemstones are classified according to their hardness, on a scale of mineral hardness called the Mohs’ scale, created by Friedrich Mohs in 1812, and based on
widely available minerals. The hardest mineral is diamond at 10, and the lowest is talc at 1 on the scale. Gemstones with a hardness of 7 and over are the best
gemstones for jewellery, but softer stones are used, provided the setting protects the stone. More care is needed with jewellery set with softer stones to avoid
scratching by harder materials.
The craft of cutting and polishing gemstones, from rough material, is performed by skilled lapidaries. The cabochon form, in which the stone is domed, with a
flat back, is the oldest known form of cutting. Transparent stones are usually cut by polishing ‘facets’ at regular intervals, at planned angles to allow optimum
reflection of light. Opaque stones are more commonly cut as cabochons, to show the stones’ colour or surface properties. Well-known optical effects are brought
to life by cutting, including chatoyancy (cat’s eye effect), asterism (star effect) and schiller (iridescence).