by Don Clark CSM
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!
While this saying may lead you to believe that diamonds are not very hard, they are actually the hardest material in the gem world. 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 that 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 variety of what we consider to be hard substances. Hardness depends on the bonds that hold 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 [Example: It scratches window glass]
6 – Feldspar [Example: A steel file will scratch it]
5 – Apatite [Example: A knife will scratch it]
4 – Fluorite [Example: A knife will scratch it]
3 – Calcite [Example: A copper coin will scratch it]
2 – Gypsum [Example: A fingernail will scratch it]
1 – Talc [Example: A fingernail will scratch it]
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. So a ruby cannot scratch a sapphire and vice versa. 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, though the curve climbs sharply upward at corundum. I’ve also included some examples of other items you might be familiar with to illustrate.
Relating this back to the old saying above, a diamond is both hard, as defined by the scientific community, and brittle, which is why it may shatter into thousands of pieces if struck by a hammer.
Hardness in a gemstone will determine the degree to which it will show wear, which is often referred to as wearability, which is why hardness and wearability go hand in hand. An opal, for example, which is quite soft for a ring stone, rapidly becomes covered with fine scratches in daily use and its 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 hardness is reported when 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 is not 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 affect 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 gem’s durability is how the gem is worn. Ring stones are subjected to considerable abuse. The simple activity of reaching into your pocket or purse, for example, 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, the damage mounts up quickly.
While rings are the most popular way to wear gems, those worn on other parts of the body do not get anywhere near the abuse a ring stone will. So if you want to own and wear 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, consider if it can be reserved for dress wear, rather than an everyday piece of jewelry as this can significantly reduce the amount of damage that accumulates over the years. Second, consider the setting. Choosing a Tiffany setting that holds the gem well above the finger with a few prongs is asking for trouble because the stone itself will come into contact with a variety of materials that may scratch it, even during occasional wear. Instead opt for a setting that surrounds the gem with a lot of metal so it is more protected. 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 gems 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.