For all these purposes it is pulverized in large iron mortars, or in steel mills; and is afterwards separated, according to the several degrees of fineness that are required, by washing it in water, and suffering the grosser particles to deposit themselves. By this operation the finer particles, which remain suspended in the water, and which are obtained by decanting the water off, and suffering it to stand for a considerable time, are separated. The particles first deposited are again ground, and again agitated in the water, to separate the finest. By these successive operations the emery is reduced to a powder so fine that, when rubbed between the fingers, it communicates no sensation whatever of grittiness. In general those particles only of the emery which remain suspended in the water, after it has stood about half an hour, are used to polish metals.
59. ADAMANTINE SPAR, or IMPERFECT CORUNDUM, is a very hard and nearly opaque stone, which varies much in colour, but is chiefly grey, with a greenish, brown, or bluish tint.
It is usually found in the form of six-sided prisms, but it sometimes occurs in shapeless masses, has a foliated texture, and is about four times as heavy as water.
The name of adamantine spar was given, by the British lapidaries, to this substance from its hardness being nearly equal to that of the diamond. It was originally discovered among the granite rocks of China; but it has since been found, and in greater purity, in Bengal and Ceylon.
In a powdered state this substance has long been used by the artists of India and China for the cutting and polishing of precious stones, and even of the diamond; but, though it will in some degree operate upon that gem, it is not sufficiently hard to bring out the peculiar beauty of it in a degree at all comparable to that which is effected by the European lapidaries with diamond powder. The Chinese also use adamantine spar for polishing steel, and in the composition of the finer kinds of porcelain or earthenware. For the cutting of seals and precious stones European workmen consider it preferable to emery; but, for minute engraving, it is much inferior to diamond powder.
60. CHRYSOBERYL is a gem of yellowish or brownish green colour, harder than quartz ([76]), and sometimes transparent; but often only semi-transparent, in which case it exhibits a bluish light, floating in the interior of the stone.
It is usually found in rounded pieces, but is sometimes crystallized in compressed six-sided prisms, and in double six-sided pyramids.
So little is this gem in request in Europe, that it is seldom to be found in the possession of jewellers; but in Brazil it is considered inferior only to the diamond. It is usually procured from South America; yet it occurs in Saxony; and, with the ruby and sapphire, amongst sand in the rivers of Ceylon.
Such is the hardness of the chrysoberyl, that, when properly polished, which is a difficult operation, it is capable of receiving a lustre nearly equal to that of the diamond. We are informed that, a few years ago, a considerable number of these gems were imported into this country from Brazil, but that the greater part of them were entirely spoiled by inferior workmen, and that the rest were so ill-cut that they remained unnoticed, and without value. The smaller stones are said to appear to most advantage in circular ear-drops; and the larger specimens form necklaces and ring stones of great beauty.
The variety which exhibits an opalescent appearance, or presents a bluish light, undulating as it were in the interior of the stone, and changing its situation according to the position of the observer, is chiefly valuable as an article of curiosity: the transparent kind is always preferred by the jeweller.