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Friday, January 15, 2010

What Is The Mohs Scale Of Hardness And Why Do You Care?

In the gem world we frequently reference the Mohs scale of hardness. But what does that scale really mean? Basically, it helps us identify one gemstone from another.

How Hard Are Natural Gemstones?
The Mohs scale was developed by Friedrich Mohs in 1812, long before there were exact scientific tools to determine hardness. It is a relative linear scale. It was based on what scratches what; the one that scratches the other is harder. If both scratch each other they are of the same hardness. The Mohs scale uses half numbers for in-between hardness. Please don’t test which gemstone scratches which on your gemstones. Instead, use this as a reminder not to “throw” your fine jewelry into a box where everything knocks together. It’s best to store all gemstones (even when set) where they can’t be scratched by a harder gemstone.

Because the Mohs scale is relative and not precise you might not be aware that a diamond is 4 times harder than corundum (sapphire / ruby) or that corundum is twice as hard as topaz. Diamond is the hardest substance found in nature. But that doesn’t mean it is indestructible. If it receives a sharp blow in one of the four directions of cleavage it can “split”. Diamonds can be chipped if hit on an edge at just the right angle. Just ask my client who dropped her new diamond ring on the pavers in her back yard. It wasn’t a big chip, but it did chip.

Below is the Mohs scale side by side with an absolute hardness scale

  Mohs Scale

Absolute Scale

  1. Talc

  2. Gypsum

  3. Calcite

  4. Fluorite

  5. Apatite

  6. Orthoclase (feldspar)

  7. Quartz / Tourmaline

  8. Topaz

  9. Corundum (ruby / sapphire)

  10. Diamond

      1 Talc

      3 Gypsum

      9 Calcite

    21 Fluorite

    48 Apatite

    72 Orthoclase (feldspar)

  100 Quartz / Tourmaline

  200 Topaz

  400 Corundum (ruby / sapphire

 1600 Diamond


How does the Mohs scale relate to day to day items? A fingernail is 2.5, pure gold is 2.5 – 3, silver is 2.5 – 3, a US coin is just under 3, platinum 4 – 4.5, a knife blade and glass are 5.5, and a good steel file is 6.5 and hardened steel is 7.

To find the gemstone that will work the best for you click here to visit AfricaGems.com.

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