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Gemstone Basic Facts

     Gemstones have several areas that a collector needs to be knowledgeable about. Hardness determines usually refers to scratch hardness, then to cutting resistance. A Viennese mineralogist Friedrich Mohs came up with this term and so became the scale of Mohs hardness scale. A comparison scale using 10 minerals of different degrees of hardness was developed and is still often used today. The 10 materials were 1 talc, 2 gypsum, 3 calcite, 4 fluorite, 5 apatite, 6 orthoclase, 7 quartz, 8 topaz, 9 corundum, and 10 diamond. #1 being the softest and #10 the hardest. Numbers 1 and 2 can be scratched with a fingernail.

    Many gemstones can be split along certain flat planes, which the expert calls cleavage. Cleavage is related to the lattice of the crystal. Lapidaries and stone setters must take account of the cleavage. Often a small tap or too much pressure when testing for hardness is sufficient to split the stone. When soldering, the temperature can cause fissures along the cleavage planes, where eventually the gem may break completely along this line. Cleavage is used to divide large gem crystals or remove faulty pieces.

    Density and specific gravity is measured to indicate the ratio of the weight of a specific material to the weight of the same volume of water. In regards to the heaviness of a gemstone, it is spoken of as weight and not the density. The density of gemstones varies between 1 and 8. Values under 2 (ex. amber) are considered light, those 2 - 4 are normal (ex. quartz), and those over 4 are considered heavy. The more valuable gemstones (such as diamond, ruby, and sapphire) have densities much greater than the more common rock forming minerals, especially quartz and feldspar.        

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