Mohs Hardness Scale
Talcreference scale from 1 (talc) to 10 (diamond).
The Mohs scale ranks resistance to scratching: 1 talc, 2 gypsum, 3 calcite, 4 fluorite, 5 apatite, 6 orthoclase, 7 quartz, 8 topaz, 9 corundum, 10 diamond.
A mineral scratches another of lower or equal hardness. Common glass is about 5.5; a steel nail is near 5–5.5; a copper coin near 3.