Mohs hardness
The ten-point scratch test that orders the mineral world.
Mohs hardness is a scratch test, nothing more: a mineral scratches anything softer than itself and is scratched by anything harder. It's a ladder of ten, and every rung has a name you already know.
The ten
1 talc 2 gypsum 3 calcite 4 fluorite 5 apatite 6 feldspar 7 quartz 8 topaz 9 corundum 10 diamond
Field shortcuts
Your fingernail is 2.5, a copper penny is 3.5, a steel knife is 5.5, and glass is 5.5. Scratch glass with the stone: if it won't, the stone is softer than quartz — almost certainly limestone or marble. If it does, you're holding quartzite or granite.