Hardness
Hardness may be defined as the mineral’s resistance to abrasion or scratching. It is measured by comparing a mineral with Moh’s scale, a set of ten minerals arranged in the order of increasing hardness, as follows:
1 [talc] 2 [gypsum] 3 [calcite] 4 [fluorite] 5 [apatite] 6 [feldspar] 7 [quartz] 8 [topaz] 9 [corundum] 10 [diamond]
A set for measuring hardness may be purchased from any dealer in mineral supplies. For rough determination, as in the field, the following objects have the hardness indicated; the finger nail 2¼, a penny 3, a knife blade about 5.5, and glass not over 6. In testing, a mineral is harder than the one it will scratch, and softer than the one by which it is scratched. For instance, if a mineral will scratch calcite and is scratched by fluorite, it is between 3 and 4 in hardness, say 3.5. When two samples mutually scratch each other they are of equal hardness. Care must be used in determining hardness, especially with the harder minerals; for often, when testing a mineral, the softer one will leave a streak of powder on the harder one, which is not a scratch. One should always rub the mark to make sure it is really a groove made by scratching.