The Physical Properties of Gemstones

Now we're going to talk about what makes up a gemstone. The physical properties of gemstones are their hardness, their specific gravity or density, and the way the break, or "cleave".  These properties all depend on chemical bonding and the atomic structure inside the stone. Take for example diamond and graphite. Diamond is the hardest known natural material, and graphite is one of the softest, yet both are composed of carbon. It's the way in which the carbon atoms are bonded together in a diamond that gives it greater resilience and hardness. 

Hardness: One of the key quality of a gemstone, hardness can be measured by how well a stone resists scratching. Every stone can be tested and classified using the Mohs Scale of Hardness, which goes in increments from one to ten. Check out the scale I have below.

Mohs Scale of Hardness

 Intervals between numbers on the scale are not equal, particularly between nine and ten. Keep this in mind though: Hardness testing is destructive, so it should only be used when all other tests fail.


I'll talk about the Knoop Scale very quickly and make it as simple as possible. Remember when I said not all numbers on the Mohs scale are equal, especially between nine and ten? This is best shown with a Knoop Scale. Look at it below.

Knoop Scale

This scale shows the indentation caused by a diamond point when it meets the surface of a mineral. Those ten stages displayed correspond with the Mohs scale. There. Hopefully that makes sense!

Specific Gravity: The specific gravity (SG) of a gem is just another indication of its density. It's calculated by comparing the stone's weight with the weight of an equal volume of water. The greater a stone's specific gravity, the heavier it feels. 

Cleavage and Fracture: Gemstones break in one of two ways. Either they cleave or they fracture. Which way they break is depends on the internal atomic structure of the stone. Gems the cleave tend to do so along planes of weak atomic bonding (cleavage planes). These planes are usually parallel, perpendicular, or diagonal to the faces of the crystal (as both are related to the stone's atomic structure). Gemstones may have one or more directions of cleavage which can be defined as perfect (almost perfectly smooth), distinct or indistinct.

When a gemstone breaks along a surface that is not related to its internal atomic structure, that's when it's referred to as a fracture. Generally, fracture surfaces are uneven. Each one has it's own descriptive name. Check them out with the pictures I've provided below. They'll give you a better idea what I'm talking about.

Perfect cleavage in Calcite

Distinct cleavage in Epidote

Indistinct cleavage in Aquamarine

Uneven fracture in Dumortierite

Conchoidal fracture in Obsidian

Hackly fracture in a gold specimen

Splintery fracture in Nephrite

Okay, well that covers the physical properties part of this journey. I did my best to keep it brief and as easily understandable as possible. I know these more scientific and technical parts get a little tedious, but if you really stop to think about it (and if you're in love with gems the way I am), the science behind the gemstone is what makes it so beautiful. Without this particular knowledge, we couldn't fully understand and appreciate gemstones for the incredible miracles of nature they are. So hang tight, we're moving right along. Next time we'll go over the crystalline shapes of gemstones. If you remember anything about Geometry, that section will make sense. So thanks for reading, and until next time!