72	PRECIOUS STONES.
	once dry, remain constant in weight. But the carat is not of absolutely the same significance in all countries. The European Diamond trade is now centred in Antwerp, and Amsterdam ; some factories having been also established in the United States of America. In Diamond-cutting the first rough formation is termed "brutage"; the rough stones being then embedded in liquid Aluminium, within moulds. Having become thus fixed when cool, they are taken from the moulds, and cloven with a circular saw, into the cutting edge of which some diamond dust with oil is forced ; next one stone is rubbed against another until the required formation is achieved; each stone is first fixed in cement for this purpose,—the cement being then set by plunging it into cold water. The " diamond-dust " which is produced by thus rubbing the two stones together falls into a copper box, called an "egrisoir," placed immediately below the diamonds which are being worked. This dust is carefully preserved. Finally a stone is polished into facets, on a revolving disc of soft steel, rotating horizontally at a speed of some three thousand revolutions to the minute. In olden times Diamond cutters contented themselves with thus producing as large a number as possible of small facets on the surface of a stone. But since then it has become manifest that for obtaining the best results, and the most sparkling lustre, a Diamond must be cut into a regular form, so that one facet-surface may reflect on another, thereby bringing out the hidden light, and beauty of the stone. Because the molten metal within a mass of which the stones are fixed for polishing is composed mainly of lead, therefore the exceedingly rapid revolution of the wheel makes this