is
impossible for gem purposes, although of great value industrially;
inferior translucent gpecimens serve for pivot supports of watches and
other delicate machines and the opaque as an abrasive; thus common
corundum is used for cutting and polishing gem minerals lower in the
scale of hardness than the diamond, a variety of it being the common
compact black emery powder used for sharpening and polishing in
mechanical and domestic uses, and familiar to everyone.
A
chemical analysis of a fine specimen of an " Oriental ruby," of the
approved rich deep red hue was as follows: alumina, 97.32; iron oxide,
1.09; silica, 1.21; in all, 99.62. The extent to which crystallography
goes and its fine, yet plain, distinctions, in determining gem
minerals, are illustrated by the marked crystal-lographic differences
between the ruby and the sapphire, which differ but slightly in
chemical composition, having the same constituents but different
proportions; thus one typical sapphire analysed entire exhibited
alumina, 97.51; iron oxide, 1.89; and silica, 0.80; in all, 100.20. The
forms of corundum generally occur in two different habits represented
by the ruby and the sapphire; in the former the prism
6
