A precious stone is a small, rare, hard stone which has inherited from Nature the name of beautiful." Thus Piero Aloisi, in his classic treatise on gems of fifty years ago, quoted Anselm de Boodt, seventeenth century scholar and physician to Emperor Rudolf II of Hapsburg.
A precious stone should be a mineral, that is, an object formed spontaneously in nature, without human intervention. This property is essential to our definition, because many modern artificial stones are highly prized, and the synthetic varieties are sometimes virtually indistinguishable from the natural ones. Beauty is essentially a subjective concept, even if the appreciation of precious stones is commonly based on objective criteria, above all optical characteristics, such as dispersive power (the so-called play of colors), color, transparency, and high refractivity. Rarity too is a criterion which has more to do with the beholder than the beheld. It is connected to that part of human nature that prefers things that are hard to come by, partly to arouse in others a sense of envy. Despite their intrinsic qualities, no one would wear rubies for ornamentation if they were as common as pebbles on the beach.
The remaining two properties hardness and chemical resistance are truly objective because they are physical and chemical. Hardness is fundamental to a precious stone; scratching of the surface or abrasion of the edges would spoil its appearance. Similarly, poor chemical resistance would eventually lead to partial disintegration, depriving the stone of value by destroying its brilliance.