DIAMOND HARDNESS
The diamond is the hardest natural material in the world. However, it is also fragile. Scientifically speaking, hardness measures the ability to resist scratches, nothing more. The overall wear grade of a gemstone takes into account hardness. However, this is just one of many factors to consider.
Did you know that the dust particles you see floating in the air and settling on tables contain quartz? This mineral has a hardness of 7 to 7.5 on the Mohs scale. This means that the quartz contained in the dust makes these particles hard enough to remove the finish from your car and the polish from your table. Dust can even cut glass. Such daily risks make the hardness of gemstones an important factor to consider when designing and wearing jewelry.
You may know that the diamond, with a hardness of 10, is the hardest material in the world of gemstones. However, there is an old saying among gemologists:
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. If you hit a piece of jade with a hammer, it will ring like a bell!"
How is this possible if the diamond is the hardest stone?
What does the hardness of gemstones mean?
The hardness of gemstones is a very misunderstood property. The word "hardness" has a very specific scientific meaning in gemology that differs greatly from its everyday use. The scientific definition of hardness is the ability to resist scratches, nothing more. If you ask most people, they will tell you that feathers are soft and glass is hard. In the world of gemology, however, glass is rather soft. A variety of what we, gemologists, consider as hard materials, could easily scratch it.
Hardness depends on the bonds that hold atoms together in a crystalline structure. This bond is evident in the ease with which layers of atoms from 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 groove, or a scratch. This represents the breaking of millions of atomic bonds on a microscopic scale. The hardness of a mineral is, more precisely, its "scratchability". Each mineral can be classified based on the other minerals it can scratch.
Mohs hardness scale
In 1812, the mineralogist Friedrich Mohs (1773-1839) established a reference scale of ten common minerals, ranked in order of increasing hardness. This scale, illustrated below, is called Mohs' hardness scale.
Each of these minerals can be scratched by the one above it and will scratch those below it. Minerals of the same hardness do not scratch each other. Thus, a ruby cannot scratch a sapphire and vice versa. A diamond is much harder than corundum, even though they are only separated by one division on the scale. Mohs' scale starts approximately linearly, but the curve rises sharply at the upper end. Corundum (9) is twice as hard as topaz (8), and diamond (10) is four times harder than corundum. (Note: only diamonds can scratch other diamonds).
Hardness ranges
The hardness of a material can vary slightly depending on its composition and also its state of aggregation. Measuring the hardness of a gemstone can be tricky. Often, a mark that looks like a scratch is actually a streak of powder left by the supposedly harder material! Fractional hardness scores, like 5.5, are reported when the literature has indicated an intermediate value. However, it's not really important whether a mineral's hardness is 5 or 5.5. A hardness range is much more meaningful.
The values indicated on this website represent all the values encountered in the literature. In one case, the disthene, the hardness of a mineral varies very widely, even within a single crystal. In most cases, the indicated hardness range is very small (one unit).
Tenacity
To return to the old adage, a diamond is both hard, scientifically defined, and brittle. The steel of a hammer (hardness 5 or 6) will not scratch a diamond, but it can break it.
"Toughness" measures a material's resistance to blows. For gemstones, these measures are usually given in descriptive terms. It turns out that most gemstones would be considered "fragile". There are a few exceptions. Cryptocrystalline quartz, like chalcedony, has "hard" toughness. Jade, whether it's jadeite or nephrite, has "very strong" toughness.
What does the wearability of gemstones mean?
Assuming you don't regularly hit your jewelry with a hammer, scratches are a more common risk. Think about the number of times you put your hands in your pockets, handbags, glove compartments, and desk drawers. Now think about what brushes against a ring when you do these things.
The hardness of gemstones greatly contributes to the degree of wear of a gemstone. We often talk about wear or sometimes "durability". An opal with a hardness of 6 will quickly be covered with fine scratches and lose its polish if it is worn daily as a ring stone. Remember that quartz (hardness 7) is a component of dust and one of the most common minerals on Earth. Just wiping the dust off a softer material can scratch it. Scratches can be tiny, even microscopic, but they accumulate and become visible over time. In contrast, a ruby with a hardness of 9 will remain shiny and lustrous for years because it is harder than most abrasive particles that contribute to wear.
Wear grades
In practical terms, stones softer than quartz will lose their polish and become dull simply by cleaning them. A value of 7 or more on the Mohs scale generally indicates that a stone is hard enough for normal use in jewelry. This is a guideline, not an absolute rule. Pearls and opals are among the most popular gemstones in jewelry, but their hardness is well below 7.
Suitability for use can be classified as excellent, very good, good, poor, or display only. An "excellent" rating means that the stone can be worn in virtually any type of setting for any occasion, even daily. A stone rated as "poor", like an opal, means that its use in jewelry should be very thoughtful. You should only wear it occasionally or in a protective setting. "Display Only" stones are intended for collections or display purposes only.
Although the hardness of gemstones has a significant effect on their wear resistance, other factors also play a role. Just because a stone is hard doesn't mean it will wear well. Some gemstones are sensitive to temperature changes or common chemicals, even sweat. One of the most important factors is the cleavage of gemstones.
Cleavage of precious stones
Technically, cleavage is related to the force with which the molecules of a gemstone bind to each other. To put it in simple terms, it's a bit like the grain of wood. You can easily split a piece of wood along the grain, but crossing the grain is much more difficult. Many gemstones have cleavage planes, which vary according to the species of gemstone. Cleavage planes are defined as perfect, good, medium, or poor, depending on how easily the mineral separates along the plane. (Some gems do not have a cleavage plane).
Have you ever seen, in old movies, diamond cutters working on large diamond crystals? Carefully, they chisel the crystals by giving sharp and carefully measured blows. If they do it right, they get two perfect pieces to cut into fabulous gemstones. If done poorly, the diamond breaks.
Diamond cutters no longer have to do this, but this example illustrates well the effect of gemstone cleavage. Diamonds are the hardest substance in nature, but they also have perfect cleavage. This means they can easily split along cleavage planes. This is an advantage for cutting if it is done correctly. Unfortunately, this also means that diamonds can chip or break with wear. On the other hand, quartz is less hard than diamond, but can withstand many shocks without being damaged.
Jewelry settings
Jewelry settings are another important factor for the wearability of gemstones. Rings are perhaps the most popular jewelry use for gemstones. However, rings also receive more abuse than any other form of jewelry. Stones with a poor wearability grade (like moonstone) would be better suited to pendants, earrings, or brooches.
If you really want your tender stone in a ring, consider the following options:
● Reserve the ring for formal wear rather than daily wear. This will greatly reduce the damage that will accumulate over the years.
● Avoid a prominent ring setting. A Tiffany setting that holds the stone well above the finger using a few prongs can only invite trouble. The stone itself comes into contact with various materials that can scratch it, even if it is worn occasionally.
● Opt for a protective setting that surrounds the gemstone with plenty of metal. Gold is soft but easily polishable and quite easy to replace when it wears out. You probably can't say the same about your gemstone!
Conclusion
The hardness of a stone is not alone a measure of its ability to be worn. Factors such as toughness, sensitivity to the environment, cleavage, and setting must also be taken into account to determine how a gemstone will wear. In our individual gemstone lists, we include wearability grades in addition to hardness measurements for many gemstones.
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