Mohs Hardness Scale

What is the Mohs scale

The Mohs scale is a scratch ranking: a mineral scratches anything below it and is scratched by anything above it. The German mineralogist Friedrich Mohs published it in 1812, using ten minerals as fixed points. It measures scratch resistance and nothing else. The steps are also uneven. On absolute tests, corundum at 9 is about twice as hard as topaz at 8, and diamond at 10 is about four times as hard as corundum, as Wikipedia's Mohs scale entry explains.

Mohs scale of the 10 index minerals

The Mohs scale of the 10 index minerals is the reference every other hardness number points back to. Each mineral scratches the ones listed above it and is scratched by the ones below it. A half step such as 6.5 means a stone falls between two index minerals. Two index minerals are crystal-shop staples: fluorite at 4 and quartz at 7.

How to do a scratch test

A scratch test takes a few household objects and a hidden spot on the stone. Use these as rough reference points: a fingernail is about 2.5, a copper coin about 3 to 3.5, a steel knife blade and window glass about 5.5, and a steel file about 6.5.

Press a sharp corner of the unknown stone firmly across a flat, clean face of the known material.
Wipe the mark with a damp finger. Powder that rubs off is a streak; a groove that stays is a scratch.
Try it both ways, because the softer of the two is always the one that gets scratched.
Test on a hidden spot or a broken edge, never on a crystal face you want to keep.

Gemstone Mohs hardness scale

The gemstone Mohs hardness scale decides which stones survive daily wear. Household dust contains tiny grains of quartz, so a stone softer than 7 slowly loses its polish as dust is wiped across it. That is why soft gems end up in pendants and earrings more often than rings.

Crystal hardness chart: every crystal we sell

This crystal hardness chart ranks every crystal we sell, hardest first, so you can see which pieces need a soft touch. Several are softer than they look. Celestite is only 3 to 3.5, soft enough for a steel knife to mark easily, and it splits along flat cleavage planes. Store the soft ones apart from quartz, which will scratch anything below 7.

Hardest minerals and hardest crystals

The hardest minerals on Earth are diamond at 10 and corundum at 9, and on absolute tests the jump from 9 to 10 is bigger than the whole climb from 1 to 9. Ruby and sapphire are both corundum. Next come chrysoberyl at 8.5, then topaz and spinel at 8. Among the crystals we stock, the hardest are the little rubies in ruby in fuchsite, then the whole quartz family at 7. Hardness is not toughness, though. Diamond can split along its cleavage if it is struck at the wrong angle, while nephrite jade, at only 6 to 6.5, is famously hard to break.

How hard is quartz

Quartz is hard enough to scratch glass and steel: it sits at 7 on the Mohs scale, the line most collectors use between soft and hard. Every variety shares that 7, from clear quartz to amethyst and agate, because they are all silicon dioxide. Kyanite straddles that line. It measures about 4.5 to 5 along the length of a blade and 6.5 to 7 across it, so a knife scratches it one way and not the other. Its old name, disthene, means two strengths.