Of the materials used in the trade, the first and most important is the diamond, used only in the time-piece as an end-stone--but at the bench all-important, as a means of making the other jewels. The diamond possesses the requisite susceptibility of polish, combined with greatest hardness of any substance known; but this adamantine quality precludes its being pierced with a through hole. Considered chemically, the diamond is pure carbon,--its different varieties differing only in structure--common charcoal, its lowest--plumbago, its intermediate grade. Another variety, called the “black diamond,” or “diamond carbon,” occurs, which is interesting as being a parallel with emery, compared with crystallic sapphire. The form of diamond most in use for mechanical manipulations, is almost always crystallized; yet it will be seen that the agglomerated form of diamond carbon plays no unimportant part in jewelling. As a jewel, no use is made of the diamond, other than as an end-stone. Marine chronometers, in which the balance will weigh from five to nine pennyweights, are almost invariably furnished with a diamond end-stone, set in steel. Yet, hard as the substance is, it is often that a pivot will cut an indentation in its face. The cause of this apparent anomaly is to be found in the structural character of the gem, and its value. The lapidary, saving in weight as possible, does not care, in “Rose Diamonds,” to pay attention to the lines of cleavage. If the face of the stone makes a slight angle with the strata of the jewel, there occur innumerable small angles of extreme thinness--the pivot, coming in contact with any of these thin portions, may fracture it, and the fragment, becoming imbedded in the tempered steel pivot, becomes a drilling tool. In our experience we have had marine chronometers sent for repair, that have lost their rate so much as to become utterly unreliable from this cause alone--the pivot having produced an indentation of the stone, creating more friction, and thus destroying the accuracy of the instrument.