Gemology 101: Introduction to Gemology
Article: Mohs Scale of Common Gems
Although gemstones are hard substances, they aren't indestructible. Some can be ruined by contact with certain chemicals, while others break easily. Many are susceptible to surface scratching, which can ruin them.
The harder a gem is, the more difficult it is to scratch. If soft gems are used in jewelry, they need to be in settings that will protect them, and worn with care.
Gems are usually rated on their hardness according to the Mohs (rhymes with nose) scale. Mineralogist Friedrich Mohs developed the scale in 1812. The Mohs scale ranks minerals from one to ten, with one being the softest and ten the hardest.
Mohs used the following minerals in establishing his scale:
- Talc (soapstone)
- Gypsum (alabaster)
- Calcite
- Fluorite
- Apatite
- Feldspar
- Quartz
- Topaz
- Corundum (ruby and sapphire)
- Diamond
Each mineral can be scratched by itself or any above it. None can be scratched by any below it. For example, topaz can scratch quartz, but it can't scratch corundum. Talc can be scratched by everything, and nothing but diamond will make a mark on diamond.
Most of the commonly used gems have a Mohs hardness of 6 or greater. The hardness of some gemstones may vary slightly, depending upon the direction in which they are scratched. Hardness may also deviate according to where the stone was found. Gemstones may be assigned a range of hardness, such as 5-6 for hematite, rather than a single number. Hardness is frequently abbreviated as the capital letter H followed by a number, so that agate is described as H 7.
The numbers on the scale classify minerals only by their hardness. That is, topaz is harder than quartz, which is harder than feldspar. It does not show by how much each gemstone is harder. Fluorite has a rating of 4 and topaz has a rating of 8. This doesn't mean that topaz is twice as hard as fluorite because its rating is twice as high. It means only that topaz is harder than fluorite. Newer scales used by gemologists organize the gems according to degree of hardness. However, the Mohs scale continues to be the most widely-used ranking.
Some comparisons to everyday items may help make the rankings easier to understand. Substances with a hardness of 2 or less may be scratched by a fingernail. A copper coin will scratch anything ranked at 3 or lower. Window glass has a hardness of 5 and may be scratched by most gemstones. A steel file will scratch all substances up to 7 1/2.
Quartz is the most common of all minerals and is an element in many gemstones, including amethyst and rock crystal. Quartz is found also in rocks, dirt, and sand. This means that most dust contains particles of quartz and therefore has a Mohs hardness of 7. Nearly all gemstones can be scratched by dust. For this reason, it is very important to never rub dust off any gem when cleaning it.
Following are the Mohs hardness of some popular gems.
- 8 1/2 Chrysoberyl
- 7 1/2 Aquamarine, emerald
- 7 Amethyst, tourmaline
- 6 1/2 Peridot
- 6 Moonstone, opal, turquoise
- 5 Lapis lazuli
- 4 Malachite
- 3 Chryscolla
The organic gems, which are not minerals, are all quite soft. Because of their variety, pearls may range from 2 1/2 to 4. Amber and ivory are softest at 2 1/2. Glass imitations of gemstones are usually 6 or less.
While the Mohs scale has its shortcomings, ranking minerals on a scale of one to ten is simple way to determine the degree of care and protection needed by gemstones.
By Sandra I. Smith, Writer
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