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Optical Properties

Physical Properties

Specific gravity
Crystal systems

Synthetic Gems


THE PHYSICAL PROPERTIES of gemstones, their hardness, their specific gravity or density and they way they break, depend on chemical bonding and the atomic structure within the stone. For example, diamond is the hardest natural material known, and graphite is one of the softest, yet both are made of the same element, carbon. It is the way in which the carbon atoms are bonded together in diamond that gives it a greater hardness and resilience


Gemstones are often tested by using the Mohs’ hardness scale to determine just how hard they are. The harder minerals are more durable in that they do not scratch easily and will hold up better in jewelry. Talc is the softest mineral with a hardness of 1 and can be easily scratched with a fingernail. The gemstones with a rating of 7 or over are relatively hard. Quartz gemstones (citrine, amethyst, etc.) range in the 7's, topaz rates 8, and corundum (sapphires and rubies) are a 9 on the Mohs' hardness scale. Diamond registers a 10 and is the hardest known naturally occurring material on earth, more than ten times the hardness of corundum at 9. There is more of a spread between the gems and minerals found between 2 and 3 and between 5 and 6, however corundum is only about 10 per cent harder than topaz.



Knoop scale

This scale shows the indentation caused by a diamond point when it meets the surface of a mineral. The 10 stages corres­pond to Mohs' point


Mohs' scale of hardness

The Mohs' scale was devised by the German mineralogist Friedrich Mohs as a means of classifying the relative hardness of minerals. He took ten common minerals and put them in order of "scratchability ": each one will scratch those below it on the scale, but will be scratched by those above it.

Natural Gemstones




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