There is an old saying that if you hit a diamond with a hammer, it will shatter into a dozen pieces. If you hit a piece of quartz with a hammer, it will split in two. Hit a piece of jade with a hammer and it will ring like a bell!
Hardness is one of the most misunderstood properties of gemstones. That is 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. Hardness depends on the bonding that holds the atoms together within a crystal structure. This bonding is reflected in the ease with which the layers of atoms at a surface can be separated by applying pressure with a sample of another material. If the second material is harder than the first, it will leave a furrow, or scratch, which represents the breaking of millions of atomic bonds on a microscopic scale. The hardness of a mineral is, specifically, its “scratchability,” and every mineral can be ranked based on those others minerals it can scratch.
About a century ago, Mineralogist Friedrich Mohs established a reference scale of ten common minerals, ranked in order of increasing hardness, as follows. It is called the Mohs Scale of Hardness.
THE MOH’S SCALE OF HARDNESS
10 – Diamond
9 – Corundum (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.
In reality, a diamond is much harder than corundum, even though the scale says they are only one division apart. The Mohs scale is approximately linear from 1 through 9; the curve climbs sharply upward at corundum, however.
A mineral may be both hard and brittle, as in the case of diamond. Diamond will scratch any other known material, but a strong hammer blow can shatter a diamond into thousands of pieces. The perfect cleavage of diamond, in fact, allows it to be more expeditiously cut. Cleavage may be an initial diamond cutting operation, as opposed to the long and tedious process of sawing.
Hardness in a gemstone will determine the degree to which it will show wear. An opal, for example, which is quite soft for a ring stone, rapidly becoming covered with fine scratches in daily use, and the polish is quickly lost. A ruby, on the other hand, will remain bright and lustrous for years because the material is harder than most of the abrasive particles in the atmosphere that contribute to gem wear.
The hardness of a material may vary slightly with composition and also with state of aggregation. The measurement of hardness is very tricky and often a mark that looks like a scratch is actually a trail of powder left by the supposedly harder material! It is really not critical whether the hardness of a mineral is 5 or 5½. Fractional hardnesses are reported where the literature has indicated an intermediate value. A range in hardness is much more meaningful, and the values reported on this website represent all values encountered in the literature. In only one case (kyanite) does the hardness of a mineral vary very widely even within a single crystal. In most cases the hardness range reported is very small (one unit).
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 its polish and become dull, simply from cleaning it. For this reason 7 on the Mohs 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 our 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 dont’ 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 gems durability is how the gem is worn. Ring stones are subjected to considerable abuse. The simple activity of reaching into your 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 your stone 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 cannot 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 affected by chemicals.
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.
The IGS gratefully acknowledges Dr. Joel Arem for his contributions to the field of gemology, and for allowing us to utilize and reproduce some of his content in the article above.