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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