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Text File  |  1992-09-03  |  569b  |  15 lines
  1. Aluminium oxide, Al2O3, the hardest naturally
  2. occurring mineral known apart from diamond
  3. (corundum rates 9 of the Mohs' scale); lack
  4. of cleavage also increases its durability.
  5. Gem quality corundum is ruby (red) or
  6. sapphire (any other colour including blue).
  7. Material of lesser quality is used as an
  8. abrasive and for other industrial purposes;
  9. synthetic corundum is also used in industry.
  10. Corundum forms in silica-poor igneous and
  11. metamorphic rocks. It is a constituent of
  12. emery, which is metamorphosed bauxite.
  13. Crystals are barrel-shaped prisms of the
  14. trigonal system.
  15.