Gemstones basically classified as precious and semi-precious. That
classification goes back to the ancient Greek. Diamond, Ruby, Emerald
and Sapphire are precious gemstones, and the rest is semi-precious
gemstones. Lots of gemstones are translucent, and they have fancy
colors. Generally, they have good hardness (5 and above in Mohs scale).
Recently, Gemstones classified with their hardness, colors, chemical
substance, and crystallization.
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http://www.squidoo.com/gemstonesclassification
