Gemstone hardness and rings. Which stones survive daily wear and which belong in a pendant.

What the Mohs scale measures

The German mineralogist Friedrich Mohs introduced the scale in 1812. It ranks ten reference minerals from talc at 1 to diamond at 10, with quartz at 7, topaz at 8 and corundum at 9.

The scale is ordinal. Equal steps do not mean equal differences. Diamond is roughly four times as hard as corundum in absolute terms, even though they sit one step apart.

The scale also measures scratch resistance and nothing else. Hardness does not tell you how well a material resists breaking.

Cleavage and brittleness decide the rest

Cleavage is a plane of weakness inside a crystal. A hard stone with cleavage can split along that plane under a knock. Brittleness makes edges chip. The International Gem Society (IGS) records both for each stone.

Topaz shows the problem. IGS gives its hardness as 8 and notes perfect cleavage in one direction, so it can split under heat, vibration or uneven stress. Tanzanite rates 6 to 7, with perfect cleavage and brittleness, and IGS suggests occasional pieces or protective settings. Sphene rates 5 to 5.5 with distinct cleavage in one direction, and IGS calls its wearability poor.

Stones that suit a ring

IGS gives spinel a hardness of 7.5 to 8. It has no cleavage and breaks with a conchoidal fracture, which gives it good resistance to blows. IGS calls it suitable for everyday jewellery, including engagement rings, and says spinel needs no special cleaning.

Tourmaline rates 7 to 7.5 with no cleavage and slight brittleness. IGS states that its hardness resists everyday scratches. Spessartite garnet rates 7 to 7.5 as well.

Stones that belong in pendants and earrings

IGS recommends protective settings for zircon in rings and calls pendants, brooches and earrings safer. Zircon rates 6 to 7.5 depending on the type. It has imperfect cleavage and it is brittle, so facet edges chip and wear.

The same logic applies to sphene and tanzanite. A pendant or a pair of earrings meets far fewer knocks than a ring.

Spinel

Hardness 7.5 to 8, no cleavage. Suits a ring.

Tourmaline

Hardness 7 to 7.5, no cleavage, slightly brittle. Suits a ring.

Spessartite garnet

Hardness 7 to 7.5. Suits a ring.

Topaz

Hardness 8, perfect cleavage in one direction. Use a protective setting.

Zircon

Hardness 6 to 7.5, brittle. Use a protective setting or choose a pendant.

Tanzanite

Hardness 6 to 7, perfect cleavage, brittle. Choose occasional pieces.

Sphene

Hardness 5 to 5.5, distinct cleavage. Choose a pendant or earrings.

Cleaning without damage

IGS advises a soft brush, mild detergent and warm water for tourmaline, topaz, zircon, sphene and tanzanite. It warns against ultrasonic and steam cleaners for topaz, zircon and sphene, and for tourmaline because most tourmalines carry inclusions.

Spinel is the exception. IGS says spinels require no special cleaning or care instructions. If you want one stone to wear every day, start there.