combination. It also forms compact granular aggregates, rounded grains and water-worn pebbles. It has an im­perfect dodecahedral cleavage. Fracture: Conchoidal to uneven. Hardness = 6-1/2 to 7-1/2. Specific gravity = 3.4 to 4.3 depending upon the composition. Colours vary. All colours except blue are met with, e.g. red, brown, yellow, green and black are the more common colours and also colourless. The colours are due to the elements that enter into the composition of the minerals of the group, and also due to impurities present in small amounts. The colourless garnets may be transparent to translucent, whereas the dark coloured varieties are translucent to opaque. Lustre is vitreous to resinous. Refractive index of gem garnets vary from 1.70 to 1.94, and the dispersion ranges from 0.024 to 0.028, but in one instance andradite goes up to 0.057, which is comparable with that of diamond. When the colour is not dark, the garnets have considerable brilliancy and fire. Garnets are isotropic and have no dichroism—this fact enables rubies to be distinguished from garnet, although refractive index and specific gravity may be more or less of the same order. Composition For the whole group, the generalized formula M'3M*2 (SiO4)3, in which M' represents the bivalent elements Mg, Ca, Mn, or Fe and M" for trivalent elements as alumina, ferric iron or chromium. Six varieties of garnets have been identified and also their physical and chemical properties. Their composition grades one into the other, and so also the physical and chemical properties.