The scale is not a contraption that measures weight. It is a scale of relative hardness of a mineral which was developed by a German mineralogist named Frederich Mohs in 1812. It is routinely used by modern jewelers and gemologists to differentiate between various gemstones. So, if you are shopping for gemstone jewelry, it's a good idea to have some understanding of the Mohs scale.
The scale consists of 10 classifications from softest being #1 to hardest being #10. Talc is considered the softest and diamonds the hardest. Mohs based his scale on 10 minerals that were all readily available. The scale tells you how easily a mineral or gemstone can be scratched by others. (hence, the diamond can scratch all others, but cannot be scratched by them so it is the hardest on the Mohs scale.)
The Mohs scale is very low tech. Basically you test your unknown mineral against one of these standard minerals. Whichever one scratches the other is harder, and if both scratch each other they are both the same hardness. It's that simple. However, sometimes a mineral will fall in between the numbers of 1-10 so you might find one rated 2 1/2-3, for instance. To give you an idea in layman's terms - your fingernail is a #2, a copper penny is #3, and glass is approximately #6-7.
Here is an overview of the Scale
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