The Mohs Scale of mineral hardness was invented in 1812 by a Friedrich Mohs a German gemologist. This scale tests hardness through scratching a mineral with another mineral to determine which one is harder. A harder substance will scratch a softer one, but not the other way around. The higher the number the harder the material. This is a ordinal scale, so a substance of 9.0 hardness is twice as hard as 8.0, a diamond at 10.0 is four times harder than 8.0.
Cleaning, Handling and Storage
This scale is very helpful in determining how much abuse a gem can take. There are sometimes special exceptions, but as a general rule gemstones of a hardness of 8.0 or higher are considered to be durable. Intermediate stones are over 6.0 and up to 8.0, softer and more delicate stones are 6.0 and under. Gems that are 6.0 and under should be handled and cleaned with more care, and not exposed to chemicals or solvents.
Exceptions to the rule
One exception to the rule is the Emerald. Although it’s tough at 7.5 – 8.0, it should be treated with some care. There are almost no perfect Emeralds found, they have numerous inclusions (flaws), so their resistance to breaking is lower than what its hardness level would imply.
No matter how hard a stone may be on the chart, the quality of the stone also determines it’s hardness. For example a flawed diamond will be softer and more brittle than one that is absolutely perfect. The same is true for all other stones and organic gems* as well.