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Gem Hardness

Using the Mohs Scale to Determine Gem Hardness

I’ve talked a lot about gem hardness in other articles and it occurred to me that some readers might not know how gem hardness is typically measured and which gems fall where on the hardness scale.

In 1812 German mineralogist Frederich Mohs took ten readily available minerals of different hardnesses and rank-ordered them according to scratch resistance.

The Mohs scale is a simple ordinal ranking of the 10 minerals he selected. Because it is so simple to use and remember, the Mohs scale is still the primary method of describing mineral hardness.

The Mohs scale goes from 1 to 10, with talc being 1 and diamond being 10.

Testing a mineral’s Mohs hardness is simply done by trying to scratch a sample with another mineral of known Mohs hardness.

For example, if you have a piece of quartz for testing purposes and want to determine the hardness of another stone, scratch one with the other. If the quartz scratches your stone, your stone has a Mohs hardness less than 7; if your stone scratches the quartz it has a hardness greater than 7.

Below are some minerals, gems and metals with their Mohs ratings and some representative absolute ratings. By looking at the absolute ratings you can tell that a diamond is actually 1600 times harder than talc and four times as hard as sapphire and ruby.

Mohs Scale Ratings (Absolute Rating)

• 2.5-3 Gold & Silver
• 3 Copper (9)
• 4-4.5 Platinum (21-35)
• 6 Lapis Lazuli
• 7 Quartz, Amethyst (linear 100)
• 8 Topaz, Emerald, Aquamarine (200)
• 9 Sapphire, Ruby (400)
• 10 Diamond (1600)

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