"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.
While the notion of size is debatable (there is no reason why a large stone should not be precious, the other four characteristics rarity, hardness, natural origin, and beauty together with chemical resistance, all constitute an acceptable definition of a precious stone. A natural object (and therefore a mineral), beautiful, rare, hard, and resistant. Let us look briefly at these properties.
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 then  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 prpperties hardess and chemical resistance are truly objective because theyare physical and chemical. Hardess is fundmentalto a precious stone;scratching of then surface or abrasion of the edges wolud spoil its appearance. Similarly, poor chemical resistance would eventually lead to partial disintegration, depriving then stone of value by destroying its brilliance.
A fabric of subjective and objective properties, fact and fantasy, sensations, fashions, superstition, and really; celebrated by poets and studies by scientists, depicted by artists and worn by the fair sex, symbol of power and wealth, product of then miner's toil and then craftsman's skill a precious stone is all this  something which has defied then passage of time and will surely continue to fascinate future generations: a thing of beauty.
Fantastic origins, not to mention magical and medicinal properties, used to be claimed for precious stones; though references were usually prefaced by something like " it is said that," or " it has been observed that," making it hard to determine whether or not the authors belived what they were saying . Ice permanently frozen by intense cold (rock crystal); product of the earth's extreme aridity combinde with the sun's powerful action (hyacinth);lynexes' urine and birds' tears (amber):  these are just a few of the fantastic notions about the  origins of precious stones not differing very much, truth to tell, from those once claimed for minerals of all kinds.
Innumerable magical and medicinal properties were attributed to precious stones over the centuries: diamond gave immunity to poison and revealed infidelity; amethyst protected against drunkenness;  heliotrope stopped nosebleed and conferred invisibility ;  sapphire enabled the wearer to escape from prison. These are merely a few better known examples, but there are also countless tales of stones with mysterious name, impossible to identify.
We may  laugh at all this; but are we ourselves innocent of all trace of superstition? It is worth  recalling that at the beginning of this century the Hope blue diamond was alleged to have brought death or economic ruin to its possessors.