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PRECIOUS STONES
79
Figs, i and 2, Plate I.) are the most common forms, though not unfrequently it takes the form of a six-faced octahedron (Fig. 3, Plate I.) ; and as the faces of all the crystals are often more or less curved, or convex, these latter are almost spherical. Some are " twinned" or " macled." (Fig. 7, Plate I.) Occasionally one distinct crystal is enclosed within another.
Although the surface is generally smooth, they are some­times indented; some have a rough surface, and others have a thin gray coating like a skin.
Two forms not considered here are carbonado, a brownish-black variety very indistinctly crystalline, and bort, an imper­fectly crystallized form which is used for mechanical pur­poses. Neither of these varieties can be used as jewels, one being without brilliancy, and the other non-transparent.
The diamond has a perfect cleavage, parallel to the faces of the octahedron. This natural structure enables the dia­mond-cutter to save much material which would otherwise go to waste, and time also. It is not necessary to split all stones preparatory to cutting. Some are cut from the stone as found. Beyond the small pieces resulting from cleavages, other fragments are saved which cannot be cut to jewels. Some of these are called " splints," and are used for mechani­cal purposes or ground to powder. Still smaller pieces can be used only for grinding.
Neither acids nor alkalies act upon the diamond. It is infusible, but combustible,—the only gem that is, in fact,— yielding to fire at a very high degree of temperature, and disappearing entirely.
It is a non-conductor of electricity, but acquires positive electricity when rubbed, and retains it for some little time. Unlike other gems, it is electric in the rough also. Some become phosphorescent by exposure to the sun's rays, or to the blue rays of the spectrum alone when concentrated upon it; others do not, though it has been claimed such stones