Soft as a Baby’s Skin
Rank samples as harder or softer by scratching them with your fingernails. Once the specimens are ranked by hardness, test that softer ones cannot scratch harder ones.
On the Mohs Hardness Scale, talc or soapstone has a Mohs hardness of 1 and is even softer than your skin. Your fingernail can scratch gypsum, which has a hardness of 2, but not calcite, with a hardness of 3. Your fingernail thus has a hardness of about 2.5.