What the scale actually measures
Mohs hardness is resistance to scratching, defined by ten reference minerals: talc at 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 6, topaz 8, corundum 9, diamond 10.
A mineral scratches anything softer than itself and is scratched by anything harder. That is the whole test, and it works in the field with no equipment beyond a steel nail and a glass plate.
The steps are not equal
The scale is ordinal, not linear. The gap between corundum at 9 and diamond at 10 is far larger than the gap between talc at 1 and gypsum at 2 — by absolute indentation hardness, several times larger.
Treating Mohs numbers as though they can be averaged or subtracted produces nonsense. They rank; they do not measure.
Hardness can differ by direction
Kyanite is the standard demonstration: roughly 5 along the length of a crystal and roughly 7 across it. A single test on a single face can therefore give a misleading answer.
Where a mineral is known to be anisotropic in hardness, test more than one orientation before committing to an identification.
Hard is not the same as tough
Diamond is the hardest natural material and will still cleave cleanly if struck along the right plane. Jade is far softer and far harder to break.
Hardness resists scratching. Toughness resists fracture. Collectors who conflate the two damage specimens.