In 1812, Austrian mineralogist Friedrich Mohs needed a fast, practical way to compare how easily minerals scratch each other, no lab equipment required. What he came up with, a simple 1 to 10 scratch-test ranking, is still the standard the mineral world uses today, more than two centuries later, essentially unchanged.
How the Test Actually Works
The Mohs scale ranks ten reference minerals from softest (talc, at 1) to hardest (diamond, at 10). To place an unknown mineral, you simply try to scratch it with each reference mineral: whichever one it can scratch, and whichever one scratches it, brackets its hardness. It’s crude by modern instrumentation standards, and it doesn’t measure toughness (resistance to chipping or breaking) at all, only resistance to scratching. A mineral can rank high on hardness and still shatter easily.
The Surprising Part: It Isn’t Linear
Here’s the detail that trips people up: the Mohs scale is ordinal, not proportional. Each step just means “harder than the one before it,” not “harder by a consistent amount.” Using more precise modern hardness measurements, the jump from corundum (9) to diamond (10), one single step on the Mohs scale, represents roughly four times the actual hardness increase. Diamond isn’t slightly harder than corundum. It’s in an entirely different category, and the 200-year-old scale simply doesn’t have room to show that.
Why It Still Matters for Buying and Storing
Quartz sits at 7 on the scale, which makes it a useful practical benchmark: anything softer than quartz, calcite (3), fluorite (4), and similar, can be scratched by ordinary dust and grit, since quartz is a major component of household dust almost everywhere. That’s a real, practical reason to store softer specimens separately rather than piled together in a drawer, a point we get into further in our guide to cleaning and storing specimens. It’s also why a harder stone like quartz or amethyst holds up well to regular handling, while something like our travertine calcite piece, calcite included, deserves a bit more care on the shelf.
