Saturday, July 24, 2010

Mohs Scale of Mineral Hardness

By on Saturday, July 24, 2010 2 Comments

Minerals and Crystals Part 1 of 2
Those of us who incorporate gemstones in our jewelry designs are well aware of the Moh's scale of mineral hardness. It's a relative scale which rates how scratch resistant one mineral is against another. It is but one of several ways to define hardness and classify minerals in material science - the modern ones are much more precise.This old scale was named after the German mineralogist, Friedrich Mohs who created it in 1812 but the idea of comparing relative hardness dates back to the ancient world.



10 minerals are indicated in this scale of which diamond is rated at a 10. It was for the longest time the toughest material - only a diamond can cut another diamond. But other substances have recently superseded diamonds in the hardness category (see my past post on Diamond is not the hardest substance any more).

How does the scale work? A copper penny for instance is a 3.5 on this scale because it is not as hard as fluorite (4) but harder than calcite (3).Window glass is rated 5.5. By the way, never tell a small inquisitive boy  a diamond will cut glass. When I was a child, my mother made that mistake when she was showing us her diamond ring. My younger brother took it from her and ran over to her wardrobe mirror to test it out before she could stop him. The scratch is still there today!

Fluorite is a 4 on the scale so this gemstone in rings is not a good choice compared to much harder gemstones like topaz (8) or a corundum (9) and diamond (10). What is corundum? That's ruby or sapphire to you and me. Both are the same mineral - the red ones are rubies and everything else are sapphires. Ancient Indians considered both "twins". The type of impurities differ in each - chromium makes rubies red but sapphires contain iron and titanium. Did you know labradorite is a feldspar (6)?

Note that the Mohs scale does not have regular intervals. Corundum is twice as hard as topaz, the next one down. But diamond is not twice as hard as corundum but four times. 

True minerals are crystalline which means their atoms are arranged in an orderly fashion. In contrast, opal, amber, jet and obsidian have no crystalline structure but are still mineral-like so they called mineraloids.

How these atoms are arranged in minerals makes a difference to their hardness. For example, diamond and graphite (pencil lead) are both made of carbon atoms. But graphite is very soft because the atoms are arranged in lattice sheets which can slide whereas the atoms in diamonds are rigidly interlocked, making them harder (see diagram below)
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The precious rocks we so love were made that way via natural geological processes. In modern times, scientists have been able to make real rubies, sapphires, emeralds and diamonds in the lab using high temperature and high tech methods which yield flawless gemstones. They continually strive to do so for those minerals have important industrial uses. But even they can't make what is in tomorrow's post.

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Original Post by THE BEADING GEM
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2 comments:

  1. Such an informative post. I didn't know this much about gemstones. I choose my gemstones for their colours, textures and shapes, but I may take more notice from now on. Thank you for this post :)

    ReplyDelete
  2. You're welcome Bobbie. The hardness of a particular gemstone is relevant if you are thinking of making rings which have to take a lot of abuse and possible bracelets as well.

    ReplyDelete

 

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