Sunstone or Avanturine Felspar
Is usually a variety of oligoclase, or soda lime felspar, having golden yellow, reddish, or prismatic internal reflections, due to the presence of minute imbedded and scattered crystals of hæmatite, göthite, or mica. Some avanturine, is, however, a mixture of albite and orthoclase, and the same name is given to quartz containing brilliant imbedded micacious crystals. The green avanturine, called amazon-stone, is microcline, a felspar.
Obsidian, or Volcanic Glass
Is often nothing more than fused or vitreous orthoclase—that is, potash felspar. But obsidian frequently contains many other minerals in small quantities, such as augite and olivine; in fact, obsidian is a melted lava, and contains the various minerals of the lava melted or else associated together. Obsidian when transparent has about the specific gravity 2·4, and is softer than crystalline felspar. Black specimens of it resemble black garnet, spinel, and tourmaline, but are much more translucent in thin splinters, as well as striated and full of bubbles.
Epidote.
The various hues of olive, brownish, and pistachio green which are presented by tourmaline occur also in great measure in epidote. The latter mineral is, however, less dichroic than the former, although in some green Siberian and Brazi ian specimens an emerald green image and a yellow one may be seen in the dichroiscope. The most famous locality is the Knappenwand, Salzburg. The hardness of epidote is about 6·5, and its specific gravity 3·3 to 3·4. It occurs in oblique prisms, often much elongated. Green epidote presents in 100 parts about the following composition:
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Silica 38
Alumina 22
Ferric oxide 15
Lime 23
Water 2