Mohs Hardness Scale

You saw hardness 7 in a gemstone listing, or picked up a mineral on a hike and want to place it. The Mohs scale answers that in a refreshingly physical way: every number has a reference mineral, and each one scratches the one below it. Knowing quartz sits at 7 explains why it marks ordinary glass, and why street dust, which is mostly quartz, quietly scuffs your watch face.

Type a number from 1 to 10 here and you get the standard mineral: talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum and diamond. Decimal values are rounded to the nearest whole number, so 5.5 returns 6, orthoclase, and 2.5 rounds up to 3, calcite. The part that trips people up is that Mohs is ordinal rather than linear: the jump from 9 to 10 is enormous in absolute hardness, far bigger than from 1 to 2.

For an improvised scratch test, keep the field references handy: fingernail around 2.5, copper coin near 3.5, steel blade and glass around 5.5, hardened file near 6.5. Always scratch a fresh surface and rub the mark afterwards, because metal leaves a streak that looks like a scratch and wipes right off. Do not run this test on a faceted gem you care about. The page shows one level at a time and does not convert to Vickers or Knoop. All local.