Mohs scale
The Mohs scale is a scale of mineral hardness devised in 1812 by the German mineralogist Friedrich Mohs. It ranks ten reference minerals, from talc (1) to diamond (10), by their ability to scratch one another: each mineral scratches all those below it. Still used in mineralogy and gemmology, it also helps in everyday practice to know which stones can take water, salt or friction.
How it works
The principle comes down to a single gesture: a mineral scratches anything softer than itself. Your fingernail sits around 2.5 and a steel blade around 5.5, so you can test an unknown stone with whatever is on hand. Between talc, which a nail bites into effortlessly, and diamond, which nothing else can scratch, every step tells the story of a crystal's resistance.
What it changes for your stones
Knowing the hardness of your stones helps you care for them. A selenite (2) or a calcite (3) will scratch at the bottom of a pocket if it travels with a quartz (7), so keep the softer ones apart in a soft pouch. It is also a good reflex before cleansing a stone or preparing an elixir: the softer a mineral, the more gentleness it asks for.
Sources
- , Manual of Mineral Science (2007)