Understanding the Mohs Hardness Scale

When an identification result mentions a number between 1 and 10 next to "hardness," that's a reference to the Mohs scale — a simple, relative ranking of how resistant a mineral is to being scratched.

It's about scratch resistance, not toughness

A common misconception is that hardness means a stone won't break or chip. That's actually a separate property called toughness. Hardness only describes resistance to scratching. Diamond, at 10, is the hardest known natural material — but it can still fracture along certain planes if struck the wrong way.

The scale, roughly

1: Talc — scratched easily by a fingernail.

2–3: Gypsum, calcite — still scratched by a fingernail or a copper coin.

4–6: Fluorite, apatite, orthoclase feldspar — a steel knife or glass will scratch these.

7: Quartz — scratches glass; glass will not scratch it.

8–10: Topaz, corundum (sapphire and ruby), diamond — extremely scratch resistant.

Why it matters for identification

Two minerals can look almost the same in color and shape, but a hardness test can immediately tell them apart. It's one of the fastest, most repeatable checks in mineral identification, which is why it shows up consistently in identification results alongside composition and crystal structure.