Gemstone hardness is often thought to refer to a gemstone’s ability to withstand blows. However, in the gem world, hardness is a measure of a gem’s resistance to scratching, that is all. It is measured on the Mohs scale for mineral hardness, which uses ten common minerals as a reference for evaluating the hardness of other gems.
Quartz is a common gemstone that comes in a variety of colors. It is often used in jewelry, but it also has a range of other applications. Quartz has a hardness of 7-7.5 on the Mohs hardness scale, but, as the rest of this article will demonstrate, gemstone hardness is not the only factor that should be considered when determining the wearability of a gemstone.
As mentioned above, gem hardness is often confused with gemstone durability. A perfect example of this is the diamond. Diamond is the hardest gemstone in the world, but it is also one of the most brittle. In other words, the only material hard enough to scratch a diamond is another diamond. But hit a diamond with a hammer, and it will shatter into several pieces.
The confusion stems from the fact that the word “hardness” in gemology has a different meaning in everyday life. So, while gemologists use hardness to refer to a gem’s ability to resist scratching, most ordinary people associate hardness with rigidity or toughness. They’ll describe feathers as soft and glass as hard. To a gemologist, however, glass is a relatively soft material because there are a variety of harder substances that could easily scratch it.
You see, hardness depends on the ease with which the atoms in a crystal structure can be separated. Gemologists test the bonds between atoms by using a sample of another material to apply pressure to the surface of a gem. If the sample turns out to be harder than the gem, it will leave a scratch. Millions of atomic bonds are broken on a microscopic scale when this happens. However, evaluating gemstone hardness can be tricky. Sometimes, the mark made by the supposedly harder material is not a scratch at all but a trail of powder!
Gemologists use the Mohs hardness scale to rank every mineral based on its ability to scratch other minerals. For instance, quartz has a hardness of 7-7.5 on the Mohs Scale, making it a relatively hard substance suitable for everyday wear. On the other hand, softer gemstones like apatite, fluorite, and calcite can be easily scratched and should be avoided in gemstone jewelry.
Let’s look at the Mohs scale in more detail below.
Developed by the German mineralogist Friedrich Mohs (1773-1839) in 1812, the Mohs scale of mineral hardness comprises ten “reference minerals” ranked in descending order for hardness. Diamond, for example, is at the top of the scale and is valued at 10 for hardness, while talc is at the bottom with a value of 1. They are called reference minerals because they are used for identifying other gem specimens.
Here is a list of all ten reference minerals as they appear on the Mohs scale with practical examples to illustrate the different hardness levels:
The way the Mohs scale works is, each of the minerals on the list can be scratched by the one above it and will scratch the one below it. However, minerals of the same hardness won’t scratch each other. Thus, ruby can’t scratch a sapphire and vice versa, but a diamond will scratch both.
The Mohs scale can be described as a qualitative ordinal scale. This means that it does not generate a precise comparison between categories. In other words, the degree of hardness for each gem variety is not the same. For example, corundum(9) is twice as hard as topaz (8), but diamond(10) is four times harder than corundum, even though it is only one division apart.
Another important consideration for measuring gemstone hardness is hardness range. This accounts for slight differences in the composition of different gemstones, which can be present even between gemstones of the same species.
However, the hardness range between gemstones of the same species is negligible in most cases. For example, quartz’s hardness can range from 7-7.5. The only mineral that displays a significant variation in hardness between gems of the same species is kyanite. Kyanite is a notoriously difficult stone to facet due to its variable hardness, which can differ even within the same crystal.
Earlier, we used a diamond to illustrate the difference between scratch resistance and gemstone durability. As one of the hardest gemstones in existence, you learned that only diamonds can scratch other diamonds. However, diamonds are brittle in terms of their durability and can be easily shattered by the steel of a hammer (which has a hardness of 5 or 6). Thus, diamonds have a ” fair ” tenacity compared to other stones.
Quartz, on the other hand, is softer than diamond but demonstrates more resistance to breakage and, as a result, is described as having “good” tenacity.
Tenacity is, therefore, a measure of gemstone durability, which is evaluated according to varying degrees of toughness.
Gemstone wearability is often used interchangeably with gemstone durability. However, gemstone hardness is the main factor that determines wearability since scratching is more of a hazard when wearing fine jewelry, especially rings. Wearability, therefore, refers to the degree to which a gem will show wear.
Generally, softer gemstones are more susceptible to scratching and are less suitable for everyday wear. That being said, many of the gemstones you see in engagement rings and other fine jewelry pieces often have a scratch hardness that is well below what is considered ideal. Opal, for instance, has a scratch resistance of 5.5-6.5 on the Mohs scale and yet is often used as a ring stone. Quartz (hardness 7) is another soft gem that rapidly accumulates scratches when worn every day. Mind you, these scratches can be microscopic, but eventually, they will become visible over time.
It’s not hard to see why harder gemstones like diamonds are more popular for engagement rings. Sapphire and ruby (with a hardness of 9) are also popular choices for wedding and anniversary bands instead of softer gems like topaz and amethyst (a type of quartz).
Generally, gems ranked 7 and above on the Mohs hardness scale are considered the most suitable for everyday jewelry. However, some of the most common materials used in jewelry, such as pearl and opal, have a hardness rating of 6 or below.
Wearability for gemstones can be graded as follows:
Stones with an “excellent” grade are safe to be worn in most jewelry settings without fear of them getting scratched or damaged. In addition, these gemstones are excellent for rings, which are subject to regular wear and tear. Examples of these gemstones include sapphire and ruby.
On the other hand, a stone with a “poor” wearability grade means that you should only wear it on special occasions or in protective settings. For example, a soft gemstone like opal is better suited for earrings or pendants, although they can also be worn in rings with an inlaid setting.
Stones graded as “display only” should never be worn in jewelry and are for collections and display purposes only.
Although hardness is often a good indication of a gemstone’s wearability, other factors are also involved. For instance, some gems are sensitive to light, heat, household chemicals, high or low humidity – even sweat.
Gemstones that are affected by extreme temperature changes and thermal shock include emerald, topaz, and many varieties of quartz, while stones that are susceptible to discoloration due to prolonged exposure to sunlight are citrine and amethyst (types of quartz), pearls, and other organic gems. In addition, gemstones like opals may crack in low humidity due to their high water content.
So, just because a stone ranks high on the Mohs hardness scale doesn’t automatically mean it will wear well. Another important factor to consider in this regard is gemstone cleavage.
Scientifically speaking, gem cleavage refers to the strength of the atomic bonds in a crystal structure. Put simply, it’s similar to wood grain. Just as wood is easier to split along the grain than against it, gemstones have a tendency to break along planes where there are weak atomic bonds. Cleavage differs between gem varieties and is measured according to the following grades:
A mineral with “perfect” cleavage will split easily, while minerals with “poor” cleavage will not. Gems that lack cleavage planes altogether are classed as having no cleavage or “none”.
Gemstone cleavage is important for faceting gems because it affects the ease with which a gemstone is cut. It also indicates the risk of shattering a stone if it is not cut correctly. A good example of this is how diamond cutters split diamonds in the past. Using a chisel and a hammer, the diamond cutter would aim carefully measured blows at the gem until it would break. If done correctly, this would result in two perfect halves. If not, the diamond would shatter.
While diamond cutters no longer use this method, the effect of gem cleavage is still crucial for wearability. This is because gemstones with perfect cleavage, like diamonds and imperial topaz, are more likely to chip or shatter with wear. A mineral like quartz, by contrast, is less likely to get damaged by a hard blow even though it is ranked below diamond on the Mohs hardness scale.
If you’re set on adding a stone with poor wearability (like opal) to your everyday jewelry collection, consider wearing it in a pair of earrings, a pendant, or a brooch. Stones like these are not recommended for rings since they are more likely to get knocked around and damaged.
However, if you really want a soft stone in a ring, here are a few ways you can protect it from wear and tear:
Gemstone hardness has a direct impact on a stone’s wearability, but this is not the only factor that determines how well a gem will wear. Everyday hazards, environmental sensitivity, tenacity, cleavage, and jewelry settings all play a role in gemstone wearability.
The mineral quartz has good wearability because of its position on the Mohs hardness scale and its durability, making it an excellent choice for a range of jewelry pieces.
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