Human fingernails score a 2 on the Mohs scale of hardness.
The Mohs hardness test, developed by Friedrich Mohs, tests the hardness of unknown mineral samples against a series of known samples. The test is useful because a mineral’s hardness is very stable. In contrast, appearance can vary markedly depending on where a mineral is found. The known samples used in the test, numbered 1 through 10, include talc, gypsum, calcite, fluorite, apatite, orthoclase feldspar, quartz, topaz, corundum and diamond. Since Mohs first devised his test, others have added minerals to the scale, allowing the test to give scores like 4.5 or 6.3. There are also common, everyday equivalents to many of the minerals he used (see Resources).
1. Clean the unknown mineral. Use the brush to obtain a clean surface.
2. Scratch the unknown with one of the reference samples. If the mineral seems hard, start with one of the harder samples. If the mineral seems soft, start with a softer sample.
3. Examine the unknown sample after you’ve tried to scratch it. Use a magnifying glass if needed. If you see that you’ve made a scratch, wipe with cloth and attempt to scratch your unknown with a softer reference sample. If you did not succeed in scratching your unknown, try the next harder reference sample.
4. Repeat the scratching process until you have identified the hardest Mohs reference sample that will not scratch the unknown mineral. Your unknown’s Mohs hardness falls between the number of that sample and that number plus one.