Friedrich Mohs proposed his scratch resistance scale in 1812 with ten reference minerals: talc at 1, gypsum at 2, calcite at 3, fluorite at 4, apatite at 5, orthoclase feldspar at 6, quartz at 7, topaz at 8, corundum at 9, and diamond at 10. The scale is ordinal, not linear: a stone rated 9 is not nine-tenths as hard as a stone rated 10. Diamond at 10 is approximately four times harder than corundum at 9 by absolute hardness measurement (Vickers hardness). Corundum at 9 is approximately twice as hard as topaz at 8. The apparent simplicity of the scale conceals significant variation at the top end.
What hardness measures — and what it does not
Mohs hardness measures resistance to scratching by a pointed surface. It does not measure toughness (resistance to fracture or chipping) or durability in the broader sense. Diamond, the hardest mineral, is relatively brittle in certain orientations. A sharp blow can cleave it along cleavage planes. Nephrite jade, rated only 6–6.5 on Mohs, is one of the toughest gemstones because its interlocking fibrous structure resists fracture. Hardness and toughness are not the same property, and both matter for practical jewellery use.
Practical thresholds for jewellery
Quartz, at Mohs 7, is the minimum generally recommended for stones used in rings and bracelets that contact surfaces regularly. Quartz occurs in airborne dust; a stone softer than quartz will be abraded by ordinary environmental contact over time. This is why emerald (7.5–8) and tanzanite (6.5–7) require protective settings despite their use in fine jewellery. Tanzanite in particular is better suited to pendants and earrings than rings for daily wear. Stones rated 8 and above, topaz, spinel, corundum, chrysoberyl, diamond, are appropriate for all setting types and wear frequencies without significant abrasion risk.
Directional hardness
Some gemstones exhibit directional variation in hardness, a property called anisotropy. Kyanite shows a Mohs value of 4.5 along one crystal axis and 6.5 along another. Andalusite varies similarly. Topaz has perfect basal cleavage that means a blow parallel to the cleavage plane is significantly more destructive than the hardness value implies. Diamond’s hardness is directional. It can be polished by diamond because the cutting direction is harder than the surface being cut, and the same diamond will resist scratching in one orientation and yield to it in another.
Using hardness for identification
A hardness set, a series of reference minerals or steel points of known hardness, is one of the oldest and most basic field identification tools. If a stone scratches quartz and is scratched by corundum, it is likely topaz or a stone in the Mohs 7–8 range. Hardness testing should be used cautiously on faceted gems because it requires scratching. Always test on an inconspicuous culet or girdle edge if at all. Modern gemmological practice relies on non-destructive optical tests (RI, spectroscopy, polariscope) rather than scratch tests for identification, but understanding hardness remains essential for interpreting setting recommendations, durability advice and identification process of elimination.