THE MOHS SCALE OF HARDNESS
The Mohs Scale, devised in 1812 by German mineralogist Friedrich Mohs (1773-1839), is used to compare the relative hardness of minerals.
The 2000+ different minerals existing are grouped into 10 levels of ascending hardness (those in each succeeding group being capable of scratching those preceding). Here's a well-known mnemonic sentence for remembering the scale:
" Those Girls Can Flirt And Other Queer Things Can Do "
Groups 1-2 can be scratched by a thumbnail.
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