Saturday, December 10, 2016

What is Mohs hardness scale?

What is Mohs Hardness Scale?

When I first began working with gemstones years ago, I knew nothing about them except that diamonds were purported to be the hardest rocks on earth.  I began researching and looking for semi-precious gemstones like Jaspers, Agates, Jades, etc. and I kept seeing a reference to this thing called "Mohs hardness".  I had no idea what this meant nor did I think about or understand its importance, until...  I began breaking some very expensive gemstones.

Because I am self-taught, there was no one around to tell me that gemstones could actually be delicate.  I mean, they're "rocks" right?  Well, yes and no.  So, me being me, I began to do some research on the gemstones that I was working with.  The Mohs hardness scale kept popping up.  I needed to learn about this scale and how to use it.  I "assumed" (which you should never do when you don't know anything about what you are researching), that it was some sort of special way to measure a gemstone's weight and worth.  I was wrong, again.  Here is the basic definition of the Mohs hardness scale:

"The Mohs hardness of a mineral is determined by observing whether its surface is scratched by a substance of known or defined hardness." (https://www.britannica.com/science/Mohs-hardness)

Now, what does that mean in layman's terms?  Simple, when it comes to gemstones, precious or semi-precious, one of the most important tests used for identifying rock and mineral specimens is the Mohs hardness test.  This test evaluates the resistance of a mineral being scratched by what is called a reference mineral.  There are ten reference minerals on the Mohs Hardness Scale.  They are:

Mineral          Hardness
Talc                      1                                                
Gypsum               2
Calcite                 3
Fluorite                4
Apatite                 5
Orthoclase           6
Quartz                  7
Topaz                   8
Corundum            9
Diamond             10

This test is beneficial because most similar minerals are close to the same hardness making their hardness a reliable diagnostic property.  For example:  If mineral A can scratch mineral B, then mineral A is harder than mineral B.  But, if mineral A does not scratch mineral B, then mineral B is harder than mineral A.  However, if the two minerals are equal in hardness, then they won't effectively scratch one another or it might be difficult to see if a scratch was made.  Now, if mineral A can be scratched by mineral B but can't be scratched by mineral C, then the hardness of mineral A is somewhere between the hardness of mineral B and mineral C.

How did all of this come about?  A German mineralogist named Friedrich Mohs developed this scale in 1812.  He chose ten minerals with distinctly different hardnesses from very soft (talc) to very hard (diamond).

Now jewelry makers, mineralogists, etc., have a scientific way to measure the hardness of minerals.  This comes in very handy when purchasing gemstones for wire wrapping.  If the stone is too soft it may crack or even break completely.  Of course, I learned this the hard way.  But now, thanks to Friedrich Mohs, I know how to choose the perfect gemstones for all of my wearable art.

For lots more information and several charts of different minerals and their hardness, visit http://geology.com/minerals/mohs-hardness-scale.shtml

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