Hardness, tenacity, and stability: The gem world’s trio of durability

Dec 31, 2023

By Lauriane Lognay

One rarely wonders if there is more to the hardness of gemstones beyond what is at face value. After all, once you have the Mohs scale down to a science, what other use could this knowledge bring to your business? We understand diamond is 10, corundum is 9, and the rest follow as easily as a fish downstream (or, I suppose, a pearl in an oyster).

As it turns out, there is a world of information and misconceptions behind gemstones’ durability. Indeed, there are three distinct factors one should consider when discussing this subject: hardness, tenacity, and stability. Each of these attributes has its own importance in the jewellery business (pun intended).

This understanding can be useful when facing any sort of challenge in business—whether you’re wondering if a ruby can go into the ultrasonic, if a pearl can go into the acetone, or if you’re questioning whether or not Ethiopian white opal is stable.

Did you ever stop to wonder why a diamond is the hardest stone of all, but can still break in two with a well-placed hit? Well, wonder no more! What follows is a textbook overview of the gem world's "trio of durability."

Hardness refers to the ability of a material to resist scratching and abrasion when a sharp fragment of another substance is passed over its surface with sufficient pressure. At a scientific level, this depends on the spacing between the atomic layers and the crystal structure.

If you attempt to scratch a diamond with a sapphire, for example, the sapphire will be damaged. If you try to scratch a quartz with a sapphire, the quartz will be damaged. If you put diamonds together in a bag, the diamonds could become scratched over time (this is explained with differential hardness in the same stone, or how you can cut diamonds with other diamonds—but we’ll leave that beast for another time). Hardness relies, amongst other things, on the atomic bonds and layers of a gemstone, and if these are strong or weak.

To help classify mineral hardness, the Mohs scale was created in 1822 by German chemist and mineralogist Friedrich Mohs. He characterized 10 minerals based on their relative difficulty/easiness to scratch each other. Today—more than 200 years after its development—this handy reference tool continues to be used by jewellers and gemmologists alike.

The Mohs scale, in ascending order of hardness, is:

If you need help remembering the stones’ order, here is a fun mnemonic device I use from time to time:

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The hardness of a precious stone can be assessed using different methods. Some of these practices could, potentially, cause damage, so be sure to exercise caution, particularly when working with a client-owned gemstone or piece of jewellery.

The most well-known technique for assessment is the Mohs hardness test kit. This kit consists of picks with different hardness, which can be used to make a small scratch on a gem to see if it affects the surface of the stone or not. Depending on which pick scratches or not your stone, you can determine an average hardness.

Take, as an example, two carbon-based gemstones:

It is important to remember the Mohs hardness scale is non-linear—meaning, a hardness of 10 and a hardness of 9 do not necessarily follow each other closely. If we had to put numbers in perspective with the absolute hardness, the relative scale would, in fact, look quite different (see Chart).

Tenacity refers to the ability of a material to resist the development of a crack or cleavage when there is physical pressure or impact. If the mineral or gemstone is hammered and the result is dust or small crumbs, it is considered brittle.

Less exact than the Mohs scale, the tenacity of a gemstone is measured in words:

Jade, for example, is among the most tenacious gemstone (i.e. exceptional)—more so than even diamond (which is measured fair to brittle, against all odds). By comparison, kyanite and zircons (both of which are considered brittle) are not considered gemstones with a strong tenacity.

While tenacity is not an exact science, this measurement does explain how the hardest gemstones can be so fragile against impact. This measurement also helps us decide the best course of action when determining jewellery settings and gemstone pairings. It is also why you so often see sculptures made out of jade with very fine details, but no sculptures made out of topaz. The stone has a great resistance to impacts.

Rounding out the trio of durability, stability refers to a material's ability to withstand physical/chemical modification or alteration. This may be caused by extreme changes in temperature, heat, light, and/or chemicals.

Such changes in temperature may occur, for example, when you are soldering a ring with the gemstone still in place. Certain stones (e.g. diamond, sapphire, tanzanite, apatite, and opal) can be weak to sudden changes in temperature, and, as such, are not considered the most stable gemstones.

Likewise, humidity can also affect a gemstone's stability. A dried-out pearl stored in a safe for too long, for example, may suddenly craze and crack, as might an unstable white opal left out of water.

Light and heat can also impact stability. Specifically, extended exposure to light can cause an unstable gem can change its colour (amethyst and citrine, I’m looking at you), while amber, jet, coral, and other fragile gems can be affected by low heat.

Kunzite is an example of a gemstone with notoriously low stability. Indeed, this gem has earned the industry nickname of "night owl’’ because of its tendency to fade from pink to colourless if it stays in the sun for too long (this can occur over years or, in some cases, within just a few months).

Exposure to chemicals can also alter a gemstone's appearance or even destroy it. Elements as simple as perfume or make-up can damage a pearl, for example, while chlorine in a pool or salt in the ocean can also damage opals and turquoise. Even seemingly innocuous activities like cooking can expose gemstones to oils which could be absorbed.

Finally, certain treatments can alter a gem's stability (or tenacity). A fracture-filled gemstone will not be a tenacious as a natural one with no treatment, for instance, while a stabilized turquoise with resin will be more tenacious than its non-treated counterpart.

Peridot is an example of a gemstone considered to have good to poor stability, while garnet, iolite, and diamond have excellent stability.

There is no known chart or scale to measure the stability of a gemstone, but some have tried, like with tenacity, to measure it with words. This, however, is not as widely accurate or accepted.

Of course, even the most seasoned gem professionals cannot know all there is to know about every stone out there. I have been studying gems of all varieties for more than a decade and I still learn something new every day—new treatments, discoveries, and studies never stop! Nonetheless, if you are willing to keep learning to the best of your ability, you will no doubt be equipped with the knowledge to show your clients the best options to create their dream jewellery.

Lauriane Lognay is a fellow of the Gemmological Association of Great Britain (FGA), has an Applied Jewelry Professional (AJP) certificate from the Gemological Institute of America (GIA), studied gemmology in Montréal, and has won several awards. She is a gemstone dealer working with jewellers to help them decide on the best stones for their designs. Lognay is the owner of Rippana Inc., a Montréal-based company working in coloured gemstone, lapidary, and jewellery services. She can be reached at [email protected].