Mohs hardness testing explained, with a field kit you can build yourself

The Mohs scale ranks mineral hardness from 1 to 10 based on what scratches what, not on how tough or durable something is. The reference minerals are talc at 1, gypsum at 2, calcite at 3, fluorite at 4, apatite at 5, orthoclase feldspar at 6, quartz at 7, topaz at 8, corundum at 9, and diamond at 10. You do not need the full reference set to get useful field information. A fingernail is roughly 2.5, a copper penny is roughly 3.5, a piece of window glass or an average steel knife blade is roughly 5.5 to 6.5, and a steel file is roughly 6.5. Test on a clean, unweathered surface, not a face that has been sitting outside for years, weathering can make a surface feel softer than the true mineral. Always test from softest to hardest and look closely for an actual groove, sometimes a softer material leaves a visible powder mark that looks like a scratch but rubs off completely.