Explaining Gemstone Properties

What do you do when you are in lock down for Covid-19? For a couple weeks you feel like the world is ending and then for the next two weeks you say, “Ok, this, isn’t so bad.”  Now a year later I am saying, “time to get off my butt and back to work!”  So without further ado, here goes what I have been thinking about.  Do most people have an understanding about the physical properties of gemstones?  Until I started working with metals and stones, I had no idea.  Therefore, I thought it might be smart to explain what gemstone properties are all about. Looking at the physical properties of gemstones, one must think in terms of: hardness and specific gravity (also known as density).  Another topic is cleaving, or how a gemstone breaks.  Additionally there is the chemical structure and atomic structure to think about. With so many topics to learn, what are we waiting for!

Hardness of Stone

As mentioned above, one of the physical properties of stone is its hardness. In previous blogs, I have briefly mentioned something called the Mohs Scale. The Mohs Scale is a scale that measures the hardness of any stone by scratching it upon another another surface. The scale ranks from one to ten. One being the softest and ten being the hardest. It might be better to explain through a picture taken from the Sphaleritemineral webpage: https://sphaleritemineral.weebly.com/hardness.html

You can see how with each level moving upward the scratch test determines the hardness level. However, enough of the hardness level, lets move on to another area.

Specific Gravity or Density

The best way to describe specific gravity of a stone is to use someone else’s words. The following is from a book called Gem Stones by Cally Hall (1994). Here I will extract her words, “The specific gravity (SG) of a gem is an indication of its density. It is calculated by comparing the stone’s weight with the weight of an equal volume of water. The greater the stones specific gravity, the heavier it will feel.” For example, a small cube of pyrite (SG5.2) will feel heavier that a larger piece of fluorite with an SG of 3.18; or a ruby (SG 4.00) will feel heavier than an emerald (SG 2.71) of similar size. Gem Stones, C. Hall, 1994, p.16.

Cleavage and Fracture

Without getting too far into detail, cleavage and fracture are the two ways that gemstones break. If you are a gemstone cutter, cleavage and fracture lines are of vital importance. Before cutting a stone, understanding the internal atomic structure of the stone is key. Without getting too academic, here is the easiest way to understand the distinction between the two:

“The key difference between cleavage and fracture is that cleavage is the manner in which a mineral will break along its plane of weakness, whereas fracture is breakage of the mineral when atomic bonding is perfect, and there is no weakness. (www.differencebetween.com/difference-between-cleavage-and-vs-fracture/). The cleavage planes are a function of the gemstone’s crystalline structure. The best example of explaining this is how wood can be split easily along the grain, but will resist splitting across the grain.”

Fracture, as opposed to cleavage is arbitrary and non repeatable. It can occur anywhere on the body of a mineral. Be it in any spatial direction, with little chance of an identical fracture occurring.(www.ajsgem.com/articles/gemstone-cleavage.html)

One could go on and on it any or all of the above categories. There is way too much information out there. For example, chemical structure and atomic structure are in another whole realm by themselves. The point is, when you go to buy a gemstone, remember that part of the pricing is placed on all of these key factors. When taking this into consideration, that price tag may not be so bad after all.