Understanding the Mohs Hardness Scale

The strength and durability of a gemstone is one of the most important aspects when choosing a gemstone for your engagement ring. For your ring to last a lifetime, the ability of the center stone to withstand the rigors of everyday wear is crucial. To better understand a gemstone’s durability, we turn to the Mohs hardness scale. Let’s break down how the Mohs hardness scale can help you choose a durable gemstone and how to take care of it.

The Mohs hardness scale is a progressive scale that is used to measure the hardness of a mineral. The scale is built based on a selective group of 10 reference minerals. As minerals are discovered, especially in the field, their resistance to scratching or abrasion relative to the 10 reference minerals helps to identify them, or at least considerably narrow down the options.

German mineralogist and geologist Friedrich Mohs created the scale in 1812. Dig kits were made that carried a small sampling of each mineral on the scale for testing in the field. This scale became indispensable for geologists and miners as they worked.

Resistance to scratching and abrasion is the only characteristic that is used to measure and place gemstones on the scale. The reference minerals on the scale are numbered 1 through 10, with 10 being the hardest mineral, diamond, and 1 being the softest, talc. It’s important to note that the scale is not linear, so it’s not representative of an equal progression. A diamond is not ten times harder than talc, it's 1500 times harder. And the difference in hardness between 8 and 9 rated stones is less than the difference between 9 and 10 rated stones.

Each mineral can only scratch those beneath it on the scale, and a diamond being the hardest mineral can only scratch itself. The most popular gemstones hold the positions of 7 and above with quartz at 7, topaz at 8, corundum (the mineral species for sapphires and rubies) at 9, and diamond at 10.

In the field, any mineral substance that is found can be scratch-tested against the known minerals in the Mohs kit. To test for the hardness of a mineral, you would start with the softest mineral, talc, and use it to try to scratch the surface of the unknown mineral. If talc is harder than the unknown mineral, it will leave an etched line on the surface. If not, there will be no scratch, instead only a residue of a powder streak. Once you discover the mineral that can’t scratch your sample, you know the hardness of your unknown mineral.

The Mohs hardness scale is an important scale used to identify minerals, but it has its limits. It’s a great indicator of surface durability, but can give a false impression of overall gemstone durability. For example, an emerald has a Mohs rating of 7.5-8 and a topaz has a Mohs rating of 8. This rating measurement would put these two stones about equal. However, a topaz wears better than an emerald because of the prevalence of natural internal inclusions in emeralds that impact the gemstone’s overall durability.

Additional properties and gemstone characteristics continue to tell an important story about the durability of a gemstone, and how well that stone will hold up to daily wear. All of these factors, combined with the Mohs hardness scale rating, provide the clearest picture of how durable a gemstone will be long-term.

These are the other important properties and characteristics to consider:

Gemstone Cleavage

Cleavage describes the ability of a gemstone to break cleanly along a distinct line. This is based on the gemstone’s crystalline structure and a gemstone can have excellent, poor, or no cleavage at all. Diamonds have perfect cleavage, so if hit in a specific spot, they will split with a distinct, clean break. This can even happen from material that is softer than a diamond. While uncommon, it’s a concern that can arise as a diamond is cut and polished.

Stability

A gemstone’s stability is determined by how well it holds up to environmental conditions like heat, fluctuating temperatures, chemical treatments, and sunlight. Opals are soft and brittle with a Mohs rating of 5 to 6, but they are also very sensitive to temperature changes due to the amount of water contained in their structure. If exposed to sudden and severe temperature changes, they can crack (which is why we don't recommend opals for engagement rings). And although rubies have a hardness rating of 9, if they have been dyed with colored oils, they will quickly lose their color from exposure to the elements.

Internal Inclusions

The molecular bonds of a gemstone affect the level of internal durability a gemstone will have. Emeralds have a Mohs rating up to 8, but due to the level of natural cracks and fissure inclusions, they can easily chip or break from surface blows. In comparison, the mineral jade has a 7 Mohs rating but its molecular bonds create a strongly held together structure that quite easily withstands blows and bumps.

Gemstone Enhancements

Many gemstones are regularly subjected to treatments to enhance their color and even their overall structural integrity. But it’s important to note that not all gemstones will react in the same way to treatment. When tourmaline undergoes irradiation to enhance its color, the internal stability is not compromised. However, when sapphire undergoes irradiation, the resulting stability is poor and the color will not last.

If you want to hone in on a gemstone with extra durable characteristics, it’s best to use the Mohs scale as a starting reference point. For most engagement rings, it is best to choose a gemstone with a Mohs rating of 7 or above. Gemstones that fall into these ratings can generally withstand the stresses of everyday wear. However, there are some other questions about a gemstone’s characteristics that can help you make your decision.

First, consider if this gemstone is naturally found with a larger amount of inclusions. Emeralds are known for their inclusions and this characteristic impacts the overall integrity of the stone even though they have a Mohs rating of 7.5 to 8. Second, see whether the stone is prone to chipping and cracking. Quartz has a hardness rating of 7, but it can take a lot of hard impact without damage.

Lastly, determine whether the stone has sensitivities to temperature changes or sunlight. Amethyst rates a 7 on the Mohs scale, but an unheated amethyst can lose its natural color if it is exposed to consistent sunlight or heat. So while rule of thumb says to choose a 7 rating gemstone or above, that should be in conjunction with a good clarity grade with fewer inclusions and knowledge of any treatments or enhancements.

Your setting and shape choice can also help add durability to your gemstone. Shapes like pear, marquise, and princess cut are vulnerable to chipping and breaking with their sharp, pointed corners. And choosing a setting with a lower mount or a bezel can help ensure any vulnerabilities are protected. The Mohs hardness scale is a great tool to be aware of in identifying and defining minerals, but remember to always fill in the rest of the picture before choosing your gemstone.