We now arrive at the point where it is necessary to discuss the manufacture and re-formation of precious stones, and also to consider a few of the tests which may be applied to _all_ stones. These are given here in order to save needless repetition; the tests which are specially applicable to individual stones will more properly be found under the description of the stone referred to, so that the present chapter will be devoted chiefly to generalities. With regard to diamonds, the manufacture of these has not as yet been very successful. As will be seen on reference to Chapter II., on "the Origin of Precious Stones," it is generally admitted that these beautiful and valuable minerals are caused by chemically-charged water and occasionally, though not always, high temperature, but invariably beautified and brought to the condition in which they are obtained by the action of weight and pressure, extending unbroken through perhaps ages of time. In these circumstances, science, though able to give chemical properties and pressure, cannot, of course, maintain these continuously for "ages," therefore the chemist must manufacture the jewels in such manner that he may soon see the results of his labours, and though real diamonds may be made, and with comparative ease, from boron in the amorphous or pure state along with aluminium, fused in a crucible at a high temperature, these diamonds are but microscopic, nor can a number of them be fused, or in any other way converted into a large single stone, so that imitation stones, to be of any service must be made of a good clear glass. The glass for this purpose is usually composed of 53.70 per cent. of red lead, 38.48 per cent. of pure quartz in fine powder, preferably water-ground, and 7.82 per cent. of carbonate of potash, the whole coloured when necessary with metallic oxides of a similar nature to the constituents of the natural stones imitated. But for colourless diamonds, the glass requires no such addition to tint it. From the formula given is made the material known as "strass," or "paste," and stones made of it are mostly exhibited under and amongst brilliant artificial lights. The mere fact that they are sold cheaply is _prima facie_ proof that the stones are glass, for it is evident that a diamond, the commercial value of which might be L50 or more, cannot be purchased for a few shillings and be genuine. So long as this is understood and the stone is sold for the few shillings, no harm is done; but to offer it as a genuine stone and at the price of a genuine stone, would amount to fraud, and be punishable accordingly. Some of these "paste," or "white stones," as they are called in the trade, are cut and polished exactly like a diamond, and with such success as occasionally to deceive all but experts. Such imitations are costly, though, of course, not approaching the value of the real stones; it being no uncommon thing for valuable jewels to be duplicated in paste, whilst the