Named for the 19th century German mineralogist who created it -- the Mohs scale measures the hardness of minerals. Basically, the 10 point scale gauges the "scratchability" of stones and minerals. It is by no means a balanced scale -- the difference in hardness between 9 (ruby) and 10 (diamond) is greater than the difference between 1 (talc) and 9.
A high Mohs rating does not always mean that a gemstone is also tough. The emerald, for example, has a Mohs rating of 7.5-8.0, yet its crystal structure makes it more brittle and prone to cracking and chipping than members of the quartz family that have lower Mohs ratings. Precious metals -- gold, platinum, silver -- have a Mohs rating of about 4, which means that they can be scratched by many gemstones. Care should always be taken to store your jewelry separately.
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