The Mohs scale: how hard are the stones in your bracelets?

It was devised in 1812 by the German mineralogist Friedrich Mohs. The idea is simple: one mineral scratches another if it is harder. Mohs chose ten reference minerals, from softest to hardest: talc (1), gypsum (2), calcite (3), fluorite (4), apatite (5), orthoclase (6), quartz (7), topaz (8), corundum (9) and diamond (10).

Note that it is a ranking, not a measurement: the steps between numbers are not equal. Diamond (10) is almost four times harder than corundum (9), which is itself about twice as hard as topaz (8).

To get a feel for it: a fingernail is about 2.5, a copper coin about 3.5, a knife blade 5.5, window glass and a steel file 6.5.

These points help you understand, not test your jewellery: scratching a stone to check its hardness leaves a permanent mark.

Household and street dust contains tiny grains of quartz, hardness 7. Every time a softer stone is wiped or rubs against a dusty surface, those grains scratch it a little. Over time, the surface goes dull.

That is why stones of 7 and above (quartz, tourmaline, aquamarine…) keep their polish for years, while turquoise, lapis lazuli or howlite need more care.

Hardness measures resistance to scratching, not to knocks. Diamond, the hardest of all, can shatter under a sharp blow; jade, less hard, is very tough. Some stones, such as feldspars (labradorite, moonstone, amazonite), split more easily along a plane: that is cleavage.

For a bracelet on elastic thread, the biggest risk is not scratching but impact: a fall onto tiles, a knock against a worktop.

Store each bracelet separately in a fabric pouch: in a shared box, hard stones scratch softer ones. Take it off for sport, housework, gardening or washing up. Clean it with a soft, slightly damp cloth rather than rubbing it dry.

A bracelet mixing hard and soft stones ages unevenly: the softer beads wear a little against the others, which is normal. Our care guide covers each stone in detail.

A stone's trade name does not always say what it is. “Picasso jasper”, for example, is actually a marble, hardness 3, much softer than true jasper (6.5 to 7). Checking a stone's mineral profile before choosing a bracelet for everyday wear avoids surprises. See also stones people mix up.