The 10 reference minerals
1 talc, 2 gypsum, 3 calcite, 4 fluorite, 5 apatite, 6 orthoclase, 7 quartz, 8 topaz, 9 corundum, 10 diamond. Each one scratches the level below and is scratched by the level above. The scale is ORDINAL — diamond isn't 'ten times harder' than talc, just the hardest of these ten. The absolute hardness gap from corundum (9) to diamond (10) is bigger than the gap from talc (1) to corundum (9).
Field-test items
Fingernail ~2.5. Copper coin ~3. Steel knife / nail ~5.5. Streak plate (unglazed porcelain) ~6.5. A piece of quartz from your own collection ~7. Carrying just those four tools handles 95% of common collecting questions. If a mineral scratches glass (~5.5) but a knife scratches it, you're between 5.5 and 6.5 — probably feldspar or one of the pyroxenes.
Common pitfalls
Test on a fresh face, not a weathered crust. Use a fresh point on the test tool — a worn knife edge gives false-low readings. Brittle minerals (like stibnite) can flake instead of scratch, fooling you into thinking they're harder than they are. And remember Mohs measures scratch resistance, not toughness — quartz (7) shatters more easily than nephrite jade (6.5) because jade's interlocking fibrous structure resists fracture.