The World's Most Useful
Gem & Jewelry Monthly Newsletter
Written by Carolyn Doyle for customers of
The Dorado Company
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Usable Gems... and a little opinion.
In the October newsletter featuring a discussion of the gem topaz, I opined that topaz makes a good gemstone... in part because it has a hardness of 8 on the Mohs scale.
I have been asked to talk about the significance of hardness and the Mohs scale, so here is my response.
Minerals, and the gems cut from their best crystals can only be positively identified by chemical analysis to identify the elements present, and then by x-rays to identify the crystal structure.
The mineral beryl is a good example of this. All beryls, including emerald have a primary chemical composition of Be3 Al2 Si6 O18, or beryllium, aluminum, silicon, and oxygen.
The difference between green beryl and emerald is the presence of minute traces of chromium or vanadium. By definition, to be an emerald one of those two elements must be present.
Minerals do, however, have other characteristics that can be used to generally identify them outside a laboratory.
Some identification characteristics require instruments that most gemologists own and use.
Some other characteristics only require handling and viewing.
- Crystal Habit
- Refractive Index
Since we're focusing on hardness today, I'll ignore the other characteristics for now.
In the early 1800's a German mineralogist named Frederich Mohs developed a simple test to aid in the field identification of minerals.
Mohs worked first in mining, then taught at the university level and did mineral collection identification.
While teaching, Mohs needed to give students a simple tool to help them with mineral identifications. He knew that hardness was a reliable test that could be performed in the field.
He knew because he had seen miners doing a simple version of the test in the mines he had worked in!
Mohs developed a relative hardness scratch test for his students. He picked ten minerals that are common in the mineralogy field and probably made up field kits using inexpensive, non-gem crystals.
The ten minerals ranging from softest to hardest are:
By looking at the written scale, you would think that gypsum was twice as hard as talc… but it isn't.
The scale is relative, rather than linear. The technology didn't exist at that time to perform exact hardness measurement.
The Mohs scale is simply based on which mineral crystal will scratch another.
If a known crystal from the Mohs hardness kit would scratch an unidentified crystal, then the unidentified crystal was softer than the one that scratched it.
By this test minerals known to be harder than the test crystal were eliminated.
By using other crystals from the kit to see which would scratch or be scratched by the unidentified crystal, most minerals could be eliminated as possibilities.
Then other clues such as color and fracture could narrow the possibilities even more, quickly giving a pretty reliable field identification.
Gem Hardness Is Good
In a gemstone, hardness is a good thing. The more resistant to scratches a gem is, the better it stands up to wear in jewelry.
Some examples of gem hardness are:
- Opal = 5 to 6
- Tanzanite = between 6.5 and 7
- Amethyst = 7
- Pyrope Garnet = between 7 and 7.5
- Tourmaline = between 7 and 7.5
- Spinel = between 7.5 and 8
- Topaz = 8
- Sapphire/ruby (Corundum) = 9
There are several attributes to a good jewelry stone, and hardness is very near the top!
Don't forget, we're offering free shipping on all orders over $99.
Just deduct the shipping charge when you fill out the order form. If you forget, we'll deduct it for you.
Until next month… please, visit me anytime at AZGem.com for a good selection of colored gems at great prices.
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