60	PRECIOUS STONES.
	held there ; the other end of the clamp is formed into two vertical legs or supports of such a height that when resting on the table, with the bar of the clamp horizontal and the dop fixed, the gem mounted on the dop is about an inch above the table. The clamps are made of iron, and when in use are further weighted, as the need occurs, with lead weights. In the case of the Diamond, grinding and polish­ing occur simultaneously, and are performed on a rapidly rotating disc of iron called a " skief" or "lap." This lap is a wheel of about twelve inches diameter and one inch thick, mounted on a steel shaft running on pivot bearings ; the wheel is of porous cast-iron, and it is mounted so as to rotate in a horizontal plane an inch or so above the level of the bench. The rate of rotation is always high, and the harder the stone under treatment the greater the speed necessary to obtain a cutting effect. When the speed is high enough, a gem may be abraded by a substance of its own degree of hardness, or even one of a lower degree, though, of course, the harder the abrading agent in comparison to the gem, the more rapid the progress with the work. The usual speed may be taken as 2,000 to 3,000 revolutions per minute, which gives a peripheral cutting velocity of, roughly, 100 to 150 feet per second, or say 75 to 100 miles per hour. Power is applied by steam, gas, water, or from an electro­motor, and the spindle is driven by a small pulley running below the bench. The abrading material in the case of Diamond is always the powder of the same mineral, as no harder substance is available ; this is naturally expensive, hence the precaution taken to save all dust from the process of bruling. In the cleaving, too, such fragments as are too small to be cut for gem use are saved, and are later crushed