TWO forms of natural glass have been employed for ornamental purposes. Obsidian results from the solidification without crystallization of lava, and corresponds in composition to a granite. The structure is seldom clear and transparent, and usually contains inclusions or streaks. The colour is in the mass jet-black, but smoky in thin fragments, and occasionally greenish. Its property of breaking with a keen cutting edge, in the same way as ordinary glass, rendered it of extreme utility to primitive man, who was ignorant of the artificial substance. The refraction is, of course, single, and the refractive index approximates to 1.5o. The specific gravity varies from 2.3 to V5. The hardness is 5 on Mohs’s scale, the same as ordinary glass.
Obsidian is obtained wherever there has been volcanic activity. Vast mines of great antiquity exist in the State of Hidalgo, Mexico.
Moldavite, which differs in no respect from ordinary green bottle-glass, is of interest on account of its problematical origin. Its occurrence in various parts of Bohemia and Moravia cannot be explained as the result of volcanic agency. It may possibly be the product of old and forgotten glass factories which at one time existed on the site. Even meteorites have been suggested as the source. The physical characters are the same as those of ordinary glass : refraction single, index 1.51 ; specific gravity 2.50 and hardness 51 on Mohs’s scale. Moldavite also passes under the names of bottle-stone, or water – chrysolite. A natural glass of the same character has been found in water-worn fragments in Ceylon, and has been sold as peridot, which it resembles in colour, but is readily distinguished from it by its very different physical properties.