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International Gem Society
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...Hardness is one of the most misunderstood properties of gemstones. That’s because the word has been given a specific and limited meaning by the scientific community. This definition varies considerably from the way it is used by the general public.
...By scientific definition, hardness is “the ability to resist scratching” and nothing more. Ask most folks and they will tell you that feathers are soft and glass is hard. In the world of gemology though, glass is fairly soft. It can be easily scratched by a majority of what we consider to be hard substances.
...About a century ago a scientist named Mr. Moh created a scale to illustrate this. It is called the Moh’s Scale. :-) He chose ten minerals of varying hardness and gave them each a value. His scale looks like this:
THE MOH’S SCALE OF HARDNESS
10 Diamond
9 Corundum, (that’s rubies and sapphires.)
8 Topaz
7 Quartz
6 Feldspar
5 Apatite
4 Fluorite
3 Calcite
2 Gypsum
1 Talc
...Each of these minerals can be scratched by the one above it and will scratch the ones below it. Minerals of the same hardness won’t scratch each other. That is simple enough, but the ramifications need to be understood.
...First notice that quartz is 7 on the list. Quartz is one of the most common minerals on earth and is a major component of common dust. That means that simply wiping the dust off of any material softer than quartz will create scratches. Of course they will be tiny, even microscopic, but over a period of time they will accumulate.
...In practical terms, this means that a gem softer than quartz will lose it’s polish and become dull, simply from cleaning it. For this reason 7 on the Moh’s scale has become a standard for determining if a gem is hard enough for normal wear. This isn’t a hard and fast rule, it is more of a guideline. (See the article “What is a Gem” for more information.)
...Again, “hardness is the ability to resist scratching” and nothing more. Just because a gem is hard does not mean it will wear well. There are a number of factors that effect how well a gem will wear. One of the most important is CLEAVAGE.
...Technically, cleavage has to do with how strongly the molecules bind to each other. To put it in layman's terms, it is much like the grain of wood. You can easily split a piece of wood along the grain, but going across the grain is much more difficult.
...Many gems have “cleavage planes.” Cleavage planes are not the same in all minerals. They are defined as “perfect, good, fair or poor,” depending on how easily the mineral will separate along the plane.
...You may have seen an old movie where a diamond cutter is sweating over cleaving a large, valuable diamond crystal. Cautiously he places the chisel on the crystal, then he gives it a brisk and carefully measured blow. If he does it right, he has two perfect pieces that can be cut into fabulous gems. If he errs, the thing will shatter!
...Diamond cutters don’t have do this any more, however it is an excellent example of cleavage. Diamond is the hardest substance in nature, but it will break if struck properly by a piece of steel, which is only 5 or 6 in hardness.
...Another very important consideration of a gem’s durability is HOW IT IS WORN. Ring stones are subjected to considerable abuse. The simple activity of reaching into one’s pocket or purse can cause a ring stone to be smacked against keys, lipstick or pocket knives. When you consider all the other things we do with our hands, this mounts up quickly.
...While rings are the most popular way to wear a gem, those worn on other parts of the body don’t get anywhere near the abuse a ring stone will. If you are wanting a sensitive gemstone, consider having it mounted in a pendant, brooch or earrings.
...If you really want it in a ring there are a couple of things that can help your stone. First is to consider if it can be reserved for dress wear, rather than an everyday piece of jewelry.
...Also consider the setting. A tiffany setting that holds the gem well up above the finger with a few prongs is asking for trouble. If you are putting a delicate gem in a finger ring, chose a setting with lots of metal around the gem to protect it. Gold is soft, but easily polished and it is fairly easy to replace when it wears away. The same can not be said for your gem!
...There are other factors besides the ones we have already covered. Some are brittle, meaning they chip easily. Others, like opal are “heat sensitive.” (In truth, opals don’t mind heat, it is a sudden change in temperature that upsets them.) Many soft and porous gems, like pearls and turquoise, are effected by chemicals. And the list goes on...
...So, I hope you are beginning to understand that, in gemology, hardness alone is not a measure of durability. Several factors have to be considered together to determine how well a gem will wear. In our gem descriptions we have a category called “Wearability.” This is a summary of these qualities that is designed as a guideline for you.