The esteem in which the different kinds of precious stone are held does not by any means depend solely on their beauty, durability, or similar characters, but is influenced by various external conditions. The price demanded for precious stones is therefore fluctuating, since it is regulated, as in the case of any other objects which are bought and sold, by the laws of supply and demand. A large supply and a small demand results in a low price, and vice versa ; when the supply and demand both vary in the same direction, that is, when they both rise or both fall equally, the price remains stationary.
The supply of each kind of precious stone depends essentially on the frequency with which it occurs in nature and on the extent to which it is mined. For reasons which have already been pointed out, precious stones which are of very common occurrence even when possessed of considerable beauty are never held in very high esteem, and consequently never command a high price, the price of the cut stone often only slightly exceeding the cost of cutting. Stones on the contrary which possess the merit of rarity are much sought after and valued far more highly.

The manufacture of glass imitations of precious stones has reached a high degree of perfection. The varieties of glass suitable for this purpose are known as paste, and this name is also applied to the imitations themselves, which are often substituted for genuine precious stones. This species of fraud, which is common enough at the present d.ay, was known and practised by the ancients, and attention was drawn to it by Pliny, who gave eloquent warnings on the subject.
The manufacturer of such kinds of glass aims at producing a substance which will, as far as possible, exhibit the more beautiful and valuable characters of genuine precious stones, and which, at the same time, will be in price as far removed as possible from the latter. The method at present followed is to produce a mass of glass which shall be as clear, transparent, and colourless as possible. When a material for coloured imitations is required, the colourless glass must be again fused with some metallic oxide capable of producing the desired colour.

A cut stone, which consists of an upper and a lower portion cemented together so as to present the appearance of a single stone, is known as a doublet. There is less deception here when the two portions consist of genuine material, for example diamond, the two small stones forming together a large' and apparently single stone, which, if really single, would be of greater value than the two stones mounted separately. Such a combination of two small, genuine stones may be referred to as a genuine doublet.

This can only be effected when the cheaper stone resembles to a certain extent the more valuable one in colour, lustre, and general appearance, and if possible approaches it also in some essential character, such as specific gravity or hardness. In such cases a little special knowledge of the subject is necessary in order to distinguish between the two stones. In this way a colourlessjtopaz may be substituted for a diamond, since both stones are colourless, and both the lustre and the specific gravity of the topaz approach those of the diamond.