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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
To be considered a gemstone, a mineral should have a crystalline structure, should refract light, and should have a hardness of 6 or above. An opal does not have a crystalline structure. It has a hardness of 6. All other gemstones have a hardness of 7 or higher and have a crystalline structure. To be considered a gemstone, a mineral should have a crystalline structure, should refract light, and should have a hardness of 6 or above.