Gemstones have attracted humankind since ancient times, and have long been used for jewelry. The prime requisite for a gem is that it must be beautiful. The beauty may lie in colour or lack of colour; in the latter case, extreme limpidity and "fire" may provide the attraction.
Iridescence, opalescence, asterism (the exhibition of a star-shaped figure in reflected light), chatoyance (the exhibition of a changeable lustre and a narrow, undulating band of white light), pattern, and lustre are other features that may make a gemstone beautiful.
A gem must also be durable, if the stone is to retain the polish applied to it and withstand the wear and tear of constant handling.
In addition to their use as jewelry, gems were regarded by many civilizations as miraculous and endowed with mysterious powers. Different stones were endowed with different and sometimes overlapping attributes; the diamond, for instance, was thought to give its wearer strength in battle and to protect him against ghosts and magic. Vestiges of such beliefs persist in the modern practice of wearing a birthstone.
Of the more than 2,000 identified natural minerals, fewer than 100 are used as gemstones and only 16 have achieved importance. These are beryl, chrysoberyl, corundum, diamond, feldspar, garnet, jade, lazurite, olivine (peridot), opal, quartz, spinel, topaz, tourmaline, turquoise, and zircon. Some of these minerals provide more than one type of gem; beryl, for example, provides emeralds and aquamarines, while corundum provides rubies and sapphires.
The physical properties of gemstones, their hardness, their specific gravity or density and they way they break, depend on chemical bonding and the atomic structure within the stone.
Inclusions are internal features of gems. The may be solids, liquids, or gases that the crystal enclosed as it grew, or cleavages, cracks, and fractures that filled after the host material finished growing.
The most usual method of fashioning a gem is to cut the surface into a number of flat faces, known as facets. This gives the stone its final shape and "cut"
Finally, color is the most important factor in determining the value of gemstones and its the most obvious visual feature, but in fact it is just one of many optical properties, all of which are dependent upon light.
GIA suggests to describe colour for the colour gemstones as Hue, Tone and Saturation, where: Hue the dominant and any additional colors visible in a stone. Tone the relative lightness or darkness of a hue or color sensation. Saturation the strength, purity or intensity of the hue present in a color sensation