A gemstone is a mineral, rock or petrified material that when cut or faceted and polished can be used in Fine Jewellery . Some are organic, or come from living substances (such as amber, which is fossilised tree resin, jet, which is a form of coal and pearl, which is the nacre of living molluscs), and some are minerals that have developed over many years.

A gemstone is treasured for its beauty, so appearance is nearly always its most important attribute. Characteristics that make a stone beautiful or desirable are colour, unusual optical phenomena within the stone, such as iridescence, interesting inclusions and rarity. Not all of these characteristics have to be present to make a stone highly desirable – diamond, for example, is prized because it is the hardest naturally occurring substance known and is able to reflect light brilliantly. Diamonds are far from rare, however, with millions of carats mined each year.

Opaque and translucent stones, such as opal and turquoise are commonly cut as cabochons, to show off their colour and surface properties.

Transparent gems are normally faceted, a method which shows the optical properties of the stone's interior to its best advantage by maximising reflected light and thus making the stone ‘sparkle' more. The facets must be cut at the right angles to optimize the light reflected back to the viewer, and this depends on the optical properties of the gem. Special equipment is used to hold the stone onto a flat lap for cutting and polishing the flat facets.

Treating Gemstones

Heat Treatment
Gemstones are often treated by heat, either to improve colour or clarity, or sometimes even to change the classification of the stone - most citrine , for example, is made by heating amethyst . Aquamarine is often heat-treated to remove yellow tones and give a purer blue and nearly all tanzanite is heated to remove brown undertones and give a more desirable blue/purple colour. A significant majority of sapphire and ruby is treated with high heat to improve both colour and clarity.

Waxing / Oiling
This is done most commonly with emerald , to fill and disguise the natural fissures that each stone contains. The wax or oil is also often coloured to improve the colour of the stone as well as its clarity. Turquoise is also often similarly treated.

Radiation
This is not a very common treatment. However, it is used in some gemstones - most blue topaz acquires its colour by being irradiated to change the colour from white to blue.

Coatings
Recently, a technique has become available whereby a gemstone can be enhanced with a coating that changes the color and appearance of the gem. Topaz, for example, is sometimes treated with a layer of titanium dioxide that changes its colour to a more golden hue with some apparent iridescence. This coating is not permanent and can be damaged by scratching.

Synthetic and artificial gemstones
Some gemstones are manufactured to imitate other gemstones. The imitations copy the look and colour of the real stone but possess neither their chemical nor physical characteristics and tend not to have the durability or long-lasting attractiveness. However, it is vital to differentiate between imitation gemstones and those that have been synthetically produced but are still ‘real'. Diamonds, ruby, sapphires and emeralds which possess very nearly identical chemical and physical characteristics to the genuine article have been manufactured in labs, for example. Synthetic corundums, including ruby and sapphire, are very common and cost only a fraction of the natural stones. There is without a doubt something about the history and age of a ‘genuine', naturally occurring gemstone, however, that cannot be replaced by the synthetic manufactured variety.