Understanding the Mohs Hardness Scale

The Mohs hardness scale, developed by German mineralogist Friedrich Mohs in 1812, is one of the most fundamental tools in mineral identification. It ranks ten common minerals by their ability to scratch one another, creating a relative scale from 1 (softest) to 10 (hardest).

The ten reference minerals are: 1-Talc, 2-Gypsum, 3-Calcite, 4-Fluorite, 5-Apatite, 6-Orthoclase, 7-Quartz, 8-Topaz, 9-Corundum, 10-Diamond. Each mineral can scratch all those below it and is scratched by all those above it.

For field testing, you can use common objects with known hardnesses: fingernail (2.5), copper penny (3.5), glass (5.5), steel file (6.5). To test hardness, try to scratch the unknown mineral with a known material. The scratch test should leave a groove in the softer material, not just a powder mark.

Hardness is determined by the strength of chemical bonds in the crystal structure. It can vary with crystallographic direction — kyanite famously has a hardness of 4.5 along its length but 7 across its width.

Remember that hardness is a relative, not absolute, property. The actual increase in hardness between each step is not uniform — the jump from 9 (corundum) to 10 (diamond) represents a far greater increase in actual hardness than the jump from 1 to 9 combined.