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Sunday, May 23, 2021

The Two-Letter Difference: Conundrum And Corundum -- May 23, 2021

Some years ago I spent a lot of time trying to sort out gemstones. 

From mineral.net and webexhibits:

Corundrum:

  • best known for two of its gem varieties: rubies and sapphires
  • same minerals; different "impurities" in trace amounts
    • ruby: red
    • sapphire: not red; most think of sapphire as blue (most popular and most values sapphire, but sapphires come in a multitude of colors
  • hardness: for all practical purposes, second hardest mineral after diamond
  • unaffected by acid and most environments
  • most common forms, due to hardness, favored as abrasives:
    • translucent brown corundrum
    • emery
  • easily synthesized
  • chemical formula: Al2O3 (oxidized aluminum); analogous to oxidized iron (rust)
  • From other sites:
    • Star of India: 563-carat, a blue sapphire
    • impurities:
      • ruby: chromium
      • blue sapphire: titanium and iron
  • science:
    • in rubies, the color can be explained by crystal field theory, but in sapphires, a slightly different process, known as charge transfer, produces the color (one wonder if there is any analogy to chlorophyll)
  • found everywhere in the world
    • Australia: dominant producer of sapphires, but of lesser quality due to their very dark color
    • SE Asia: fine quality sapphires
    • Sri Lanka: sought-after cornflower blue sapphires
  • While at least 1% chromium must be present in corundum before the deep red ruby color is seen, sapphire blue is apparent with the presence of only 0.01% of titanium and iron. 

I guess we'll stop there.

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