Rarestone

← All guides

Identify · 2 min read

Mohs hardness in 60 seconds

Fluorite, 4 on the Mohs scale
Fluorite, 4 on the Mohs scale
The short answer: Your fingernail is about 2.5, a copper coin about 3, glass and steel about 5.5, and quartz is 7. If a rock scratches glass, it's harder than 5.5.

  1. Learn the scale

    1 Talc · 2 Gypsum · 3 Calcite · 4 Fluorite · 5 Apatite · 6 Feldspar · 7 Quartz · 8 Topaz · 9 Corundum (ruby, sapphire) · 10 Diamond.

  2. Build a pocket kit

    Fingernail ≈ 2.5 · copper coin or wire ≈ 3 · steel nail or knife ≈ 5.5 · glass ≈ 5.5 · steel file ≈ 6.5 · quartz crystal = 7.

  3. Scratch, then wipe

    Try to scratch the rock with each tool, softest first. Wipe the spot: a real scratch stays, while powder from the softer object rubs off.

  4. Read the result

    Your fingernail scratches it: under 2.5 (talc, gypsum). A steel nail scratches it, but it won't scratch glass: about 3 to 5.5 (calcite, fluorite). It scratches glass: over 5.5 (quartz, agate, jasper, chert).

Tips
  • Test on a hidden spot of any stone you want to keep nice.
Identify rocks on the spot with Rarestone for iPhone: point your camera at a rock and tap it. Get the app.

Photos: Fluorite (Crystal Victory Mine, near Cave-in-Rock, Illinois, USA) 1 by James St. John, CC BY 2.0. General information only: rules, safety and values vary, so check locally and ask an expert when it matters.