Jewels have always been a powerfully attractant for mankind, and the treatment of precious stones and the precious metals in which they are set, often serves as important evidence, not only concerning the art of early times and peoples, but also concerning their manners and customs. Jewels have been the gifts and ransoms of kings, the causes of devastating wars, of the overthrow of dynasties, of regicides, of notorious thefts, and of innumerable crimes of violence. The known history of some existent famous gems covers more years than the story of some modern nations.

So wide and so interesting is the subject of precious stones and precious metals, their artistic treatment apart and combined, their importance in society, commerce, and the arts, their part in the wealth of individuals and nations, that it is in a high degree remarkable that, comparatively speaking, so few books have been written about them.

Where the gemstone possesses distinctive features such as hardness, colour, transparency, refraction or double refraction of light and where the stone iss also comparatively rare the classification "precious" may be applied.  Where the gemstone iss not particularly rare or where availability is not controlled it will most likely be classified as "semi-precious"

A sudden flooding of the market with one class of stone, even if it was previously considered rare and precious, results in an equally sudden drop in the intrinsic value of the jewel to such an extent as perhaps to wipe it out of the category of precious stones.  Amethyst suffered this when the South American discoveries became available on the World market.  The opposite can also be true and in some cases a gemstone will switch from one classification to the other and back.  This was the case with the diamond where it initially enjoyed a rarity that ensured it's "precious" classification.  Eventually though diamond discoveries revealed that it was actually a quite common gemstone and it's availability consequently saw it's value fall.  De-Beers entry into the diamond market though saw the availability of the diamond controlled and it once again became a "precious" gemstone.

For a stone to have the slightest claim to be classed as "precious" it must conform to at least  several of the following requirements:

• it must withstand the action of light without deterioration of its beauty, lustre, or substance;
• it must be of sufficient hardness to retain its form, purity and lustre under the actions of warmth, and dust;
• it must not be subject to chemical change, decomposition, disintegration, or other alteration of its substance under exposure to atmospheric air;
• it must display a wonderous play of colour or fire, due to the action of light, quite apart from the colour of the stone itself;
• it must have superb transparency, colour, lustre, and/ or brilliance; or
• where the stone is opaque, or semi-opaque, the beauty must exist in the richness of colour.

In all cases the value of the stone cannot be appreciated fully until the gem is separated from its matrix and polished, and in some cases of transparent stones, cut in variously shaped facets. Other stones, such as the opal, turquoise and the like, are cut or ground in flat, dome-shaped, or other form, and then merely polished.

Broadly speaking, the greatest claim which a stone can possess in order to be classed as precious is its rarity. A rare stone though is of little value if it is unknown so to this may be added public opinion, which is led for better or worse by the fashion of the moment. The publicity given to Tanzanite by Tiffany is a case where a rare though unknown stone was raised to prominence by the publicity it received.

Natural gemstones represent the slow chemical action of water, decay, and association with, or near, other chemical substances or elements. Combined with the action of millions of years of time, and the unceasing enormous pressure  of thousands, perhaps millions, of tons of earth, rock, and the like, and subjected, for a certain portion at least of that period, to extremes of heat or cold, determines the nature of the gem.