The fashioning of coloured stones, as all the gem-stones apart from diamond are termed in the jewellery trade, is on account of their inferior hardness a far less tedious operation. They are easily slit, for which purpose a vertical wheel (Plate VIII) made of soft iron is used; it is charged with diamond dust and lubricated with oil, generally paraffin. When slit to the desired size, the stone is attached to a conveniently shaped holder by means of a cement, the consistency of which varies with the hardness of the stone. It is set in the cement in such a way that the plane desired for the table facet is at right angles to the length of the holder, and the whole of the upper part or crown is finished before the stone is removed from the cement. The lower half or base is treated in a similar manner. Thus in the process of grinding and polishing the stone is only once re-set; as was stated above, diamond demands very different treatment. Again, all coloured stones are ground down without any intermediate operation corresponding to bruting. The holder is merely held in the hand, but to maintain its position more exactly its other end, which is pointed, is inserted in one of the holes that are pierced at intervals in a vertical spindle placed at a convenient distance from the lap (Plate VIII), which one depending upon the inclination of the facet to be formed. For hard stones, such as ruby and sapphire, diamond powder is generally used as the abrasive agent, while for the softer stones emery, the impure corundum, is selected; in recent years the artificially prepared carborundum, silicide of carbon corresponding to the formula CSi, which is harder than corundum, has come into vogue for grinding purposes, but it is unfortunately useless for slitting, because it refuses to cling to the wheel. To efface the scratches left by the abrasive agent and to impart a brilliant polish to the facets, material of less hardness, such as putty-powder, pumice, or rouge, is employed; in all cases the lubricant is water. The grinding laps are made of copper, gun-metal, or lead; and pewter or wooden laps, the latter sometimes faced with cloth or leather, are used for polishing. As a general rule, the harder the stone the greater the speed of the lap. [Illustration: _PLATE VIII_ SLITTING COLOURED STONES POLISHING COLOURED STONES] [Illustration: _PLATE IX_ FACETING MACHINE]