This is just a quick (as in not indepth) discussion of the MOHS scale of hardness. The MOHS scale was developed by (and subsequently named after) Freiderich Mohs in 1812. It is one way a geologist (or a rockhound) to determine what type of stone they might have. It is based on using one stone to scratch another, and since this has all be done and recorded, I prefer to look up the stone online to see what its hardness is. However if you are in the field and want to know what type of mineral you may be looking at, you might use the MOHS scratch test.
The hardness of quartz crystal on the MOHS scale of hardness is a 7. For an excellent in-depth description of what exactly the MOHS scale of mineral hardness is, here's a website. In a nutshell, the MOHS scale is a way of rating the hardness of a mineral based on its ability to scratch the mineral below it. This is in order to determine which mineral class the stone is question might be in. Rated on a scale from 1 to 10, 1 (the softest) is Talc (think talcum powder), and 10 (the hardest) is Diamond. Each mineral on the scale is able to "scratch" the mineral below it. The scale is:
- Talc (examples: Talc, Soapstone)
- Gypsum (examples: Alabaster, Satin Spar, Selenite)
- Calcite
- Flourite
- Apatite
- Orthoclase (examples: Adularia, Moonstone)
- Quartz (examples: Amethyst, Aventurine, Chalcedony [Agates] and more)
- Topaz
- Corundum (examples Ruby, Sapphire)
- Diamond
Another way to determine or use a scratch test is by understanding and using the hardness of common objects, this list is as such:
- Fingernail, 2 - 2.5 (a fingernail can scratch talc)
- copper - 3 (a sharpened penny could scratch a fingernail)
- Nail - 4 (a nail could scratch a penny)
- Glass - 5.5 (glass could scratch a nail)
- Knife Blade - 5 - 6.5 (a knife could scratch glass)
- Steel File - 6.5 (a steel file could scratch a (non-steel) knife)
- Streak Plate (bathroom tile) 6.5 - 7 (a piece of bathroom tile could scratch a file)
- Quartz 7 (quartz could scratch a bathroom tile)
Serious rock hounders used to carry a "streak test" kit, which was a collection with one example of each of the types of stones in it to use when rockhounding (or once they got home). There is a process of how to properly "scratch" the stone in question, and then by following other clues (such as color or shape, and where they discovered the stone), the person could determine what type of stone they might have found. Now, since we have the internet, it is much easier to look it up, and they also make sets of tools, each tip with the corresponding hardness of the mineral in each category. Personally I have never been able to bring myself to scratch the surface of any stone just to determine what it is (seems so destructive!) However this is the way the MOHS scale of hardness was created.
Why would it be important to know about the MOHS scale? As just ONE example, if you typically wear stones which are soft (anywhere under quartz) more care will need to be taken when cleansing. You might not want to rub an opal necklace after a trip to the beach (or even to wear it at the beach, actually)... Sand is comprised of a great deal of quartz. So imagine what might happen to your opal (or moonstone) necklace or ring if you rubbed it if there were grains of sand on the surface? Another example is if you were going to etch runes onto a crystal or even onto a piece of glass... you would need a diamond tip bit in order to do this. Several diamond tip bits, actually!
This has just been a bit of information on rock hounding. If you have found this post interesting and would like to know more, a Google search on the MOHS scale would work, or here is a good article written by Hobart King on the website geology.com.