Marc Aronstam on: gemstone hardness versus toughness

gemstone-toughness-june-2013smallerThe more you know about gemstones, the better you understand and appreciate the gem stones in your personal jewelry collection. That’s why I’d like to blog about two properties of gem stones you often hear about: hardness and toughness. Although they sound similar, hardness and toughness of a gem are two separate, unrelated physical characteristics of minerals.

Hardness generally refers to the Scale of Mineral Hardness created by Friedrich Mohs back in 1812. Mohs believed that absolute hardness, or “scratch hardness,” is the measure of how resistant a mineral is to fracture, scratching or cutting. A diamond, for instance, gets a 10 of 10 possible on the Moh’s hardness scale. Next in line is a little known mineral called ‘corundum,’ fetching a 9 on the Moh’s Scale.

When we speak of toughness when referring to a gem stone, we mean a mineral’s ability to resist blunt force trauma such as chipping, fracturing or breaking. Most gem-grade minerals are considered brittle. Some, like opal, zircon, and kunzite, are extremely brittle. Sharp blows can cause them to fracture.

What makes a gem stone tough? Their inner structure. Microcrystalline, overlapping, interlocking crystal structures gives some gems great physical toughness with tensile strengths comparable to structural grade steel and the compressive of concrete.

When it comes to the gem stones in your collection, it pays to find out how your stones are rated on the Mohs scale and their toughness. For instance, you want to be very careful not to hit the emerald in your emerald ring. A blunt force trauma to an emerald could easily result in a fracture, ruining the stone. If that same ring held a sapphire, it might be just fine.

Knowing the qualities of your gem stones is not that tough, if you’ll pardon my pun. The deeper your knowledge of gem stones, the better owner or collector you become. Come admire some of my favorite ‘hard’ and ‘softer’ gem stone jewelry, and take home some favorites!

Posted in: Gemstones, Marc Aronstam

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