Gemstones are basically categorized based on of their crystal structure specific gravity refractive index and other optical properties such as pleochroism. The physical property of hardness is defined by the non linear Mohs scale of mineral hardness. A fine goldsmith can and will work to a tolerance approaching that of precision machinery but largely using only his eyes and hand tools. Quite often the goldsmiths job involves the making of mountings for gemstones in which case they are often referred to as Jewellers.
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