OBSIDIAN (GemRocks, Ref-1) ( Fr- obsidienne/obsidiane; Ger- Obsidian/Feuerkiesel; Nor- obsidian; Rus- )
DESCRIPTION: Obsidian is natural glass formed by quenching (i.e., rapidly cooling) magma of granitic/rhyolitic or similar composition.
Colors - commonly dark gray to nearly black, less commonly bluish gray or reddish brown and rarely with streaks of pastel hues that are pinkish, yellowish, greenish, purple, brown, etc.; some is iridescent, most commonly exhibiting silver or golden tones; some is mottled or roughly banded, the latter apparently representing flow patterns.
H. ~ 5-5½
Light transmission - most is transparent to subtranslucent in thin slivers; some reddish brown obsidian and a dark green glass, most of which is sideromelane rather than obsidian (see REMARKS) is virtually opaque.
Luster - vitreous or subvitreous
Breakage - conchoidal fracture is typical
Miscellany - some obsidian appears to be chatoyant, iridescent, or aventurescent because of the presence of minute inclusions of minerals -- e.g., hematite and/or ilmenite -- and/or bubbles that are relatively abundant in some lamellae and less so or virtually lacking in intervening lamellae. Streak -- i.e., powder -- of virtually all obsidians, even those that re nearly black, appears white.
OTHER NAMES: A few obsidian bodies have extents that have led to their being named according to the scheme used for stratigraphic units (see Appendix B, Glossary). Three examples are the East Lake Obsidian of Obsidian cliff, Yellowstone National Park, Wyoming; the Mono Craters Obsidian of the Sierra Nevada of California; and the Newberry Obsidian of Oregon. Additional names, many of which have been given in the markeplace, follow: