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Text File  |  1992-09-01  |  351b  |  10 lines
  1. A black or dark-coloured glassy volcanic
  2. rock, chemically similar to granite, but
  3. formed by cooling rapidly on the Earth's
  4. surface at low pressure. The glassy texture
  5. is the result of rapid cooling, which
  6. inhibits the growth of crystals. Obsidian was
  7. valued by the early civilizations of Mexico
  8. for making sharp-edged tools and ceremonial
  9. sculptures.
  10.