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Definition: obsidian from Philip's Encyclopedia

Rare, grey to black, glassy volcanic rock. It is the uncrystallized equivalent of rhyolite and granite. It makes an attractive semi-precious stone. Hardness 5.5; r.d. 2.4.

Summary Article: Obsidian
from Guide to Gems

Obsidian is the best known glassy rock. It is a volcanic glass and is supposedly named after Obsidian, a Roman said to have brought the first specimens from Lake Shalla, Ethiopia to Rome. The rock originates from explosive volcanoes. It is made of the same minerals as granite but cooled too quickly to crystallize. Obsidian has no crystalline structure but may contain rare tiny crystals of QUARTZ, FELDSPARS, pyroxene, or magnetite.

Being a siliceous glass, it breaks with a conchoidal fracture producing sharp slivers that must be handled carefully. Obsidian may be fashioned into a razor-sharp cutting edge, and ancient civilisations used it for jewellery, mirrors, arrowheads, spearheads, scrapers, and cutting tools, such as the sacrificial knives of the Aztecs.

Today, transparent specimens are faceted, usually into step cuts, while less transparent pieces are fashioned into cabochons or table cuts. Brilliants are also popular. Dark nodules found in Mexico and Arizona and New Mexico in the United States are called ‘Apache tears’, which Native Americans have long used for decorative pieces and jewellery, such as necklaces, bracelets, and amulets. Also especially prized as jewellery is the rare snowflake obsidian with white patches that are internal bubbles or crystals of potassium feldspar. Another favourite is the silky lustred variety with a sheen caused by minute crystals (‘crystallites’).

Occurrences in the USA include the Obsidian Cliffs in west-central Oregon, the Obsidian Cliff at Yellowstone National Park in Wyoming, the Glass Buttes in Oregon (entirely of obsidian), and other states, including New Mexico, Utah, and Hawaii. Other locations include Cumbria in England, the Lipari Islands of Italy; and Mexico, Iceland, Japan, Java in Indonesia, Hungary, Slovakia, Ecuador, and Guatemala.

Polished cabochon and heart-shaped obsidian

© 2003 Philip's

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