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In addition, it must be hard and durable. Its value increases if it is also rare.The beauty of a gem is measured in terms of its clarity, brilliance, and color. Comments
There are some substances, like opal, which have the appearance of a mineral but lack any definite internal structure, are sometimes called mineraloids. Lutgens and Tarbuck give the following list of essential characteristics of a mineral: 1  It must occur naturally.