sphene, biotite and iron oxides are the other common constituents, but these rocks show much variety of composition and structure.
Reddish grains of sphene or of garnet are occasionally visible.
The rarer stones, sphene and epidote, likewise exhibit this property markedly.
The iron oxides, originally ilmenite, are usually altered to sphene.
Most gem-stones are harder than quartz, though precious opal, turquoise, moonstone and sphene are inferior to it in hardness.