The Mohs Scale — Friedrich Mohs's 1812 Hardness Reference Standard
The Mohs Scale — Friedrich Mohs's 1812 Hardness Reference Standard
The ten-step relative hardness scale that remains the foundational hardness reference in gemmology and mineralogy
The Mohs scale is the relative hardness scale devised by the German mineralogist Friedrich Mohs in 1812, ranking ten reference minerals from talc (1) to diamond (10) according to which can scratch which. The scale is the foundational hardness reference in gemmology, mineralogy, and the broader materials sciences, and it remains in routine practical use over two centuries after its publication despite the development of more precise instrumented hardness measurements. For the gem trade, Mohs hardness is one of the basic specifications attached to every species, with direct implications for durability, jewellery design, and recommended care.
The ten reference minerals
Mohs selected ten common minerals as reference points, each chosen for its hardness and ready availability: 1 talc, 2 gypsum, 3 calcite, 4 fluorite, 5 apatite, 6 orthoclase, 7 quartz, 8 topaz, 9 corundum, and 10 diamond. Each mineral on the scale can scratch any mineral below it and can be scratched by any mineral above it. A mineral with no entry on the scale receives an estimated value by comparison with the reference set: jadeite at approximately 6.5 to 7, opal at 5.5 to 6.5, peridot at 6.5 to 7, and so on.
The scale's selection of reference minerals reflects the mineralogical knowledge of the early nineteenth century and the practical requirements of field testing. Mohs needed minerals that were widely available, that could be obtained in clean specimens, and that occupied roughly even spacings on the relative hardness continuum.
Non-linearity
The Mohs scale is relative rather than absolute, and the spacings between consecutive numbers are not equal in the way that the steps on a linear scale would be. Modern instrumented measurements — Vickers and Knoop hardness, in particular — show that the relationship is approximately exponential through the upper part of the scale. Diamond at Mohs 10 is roughly four times harder in absolute terms than corundum at Mohs 9, while corundum is only about twice as hard as topaz at Mohs 8. The non-linearity is well known and does not impair the scale's practical use, but buyers should be aware that an apparently small numerical difference at the upper end of the scale corresponds to a large difference in actual hardness.
Practical implications for jewellery
Mohs hardness has direct implications for jewellery durability. The principal practical threshold is Mohs 7: stones at or above this hardness resist scratching by ordinary household dust, the principal abrasive in everyday wear, because dust contains quartz at Mohs 7. Stones below Mohs 7 — including opal, turquoise, lapis lazuli, peridot, tanzanite, malachite, fluorite, and many others — are vulnerable to gradual abrasion in daily-wear ring positions and are best reserved for protected ring settings, pendants, earrings, and other lower-impact applications.
Above Mohs 7, durability concerns shift from scratching to cleavage and fracture. Topaz at Mohs 8 has perfect basal cleavage that makes it vulnerable to chipping at impacts despite its hardness; emerald at Mohs 7.5 to 8 is hard but brittle and fissured; even diamond at Mohs 10 can be cleaved or fractured by sufficient impact. Hardness alone does not guarantee toughness, and trade understanding of jewellery durability requires consideration of cleavage, fracture, and the mechanical environment of the intended setting.
The scale in modern practice
The Mohs scale remains in routine practical use because it is simple, requires no special equipment, and provides immediately useful information about a stone's likely durability. Field gemmologists and lapidaries assess unknown materials by attempting scratch tests with reference minerals or with calibrated hardness picks, and the Mohs estimate forms one of the basic identifying characteristics reported in standard gemmological literature.
For instrumented work — research, materials engineering, and certain quality-control applications — the Vickers and Knoop tests have largely replaced Mohs as the primary hardness measurement. The Mohs scale, however, continues to be the standard reference taught in introductory mineralogy and gemmology courses and the standard hardness specification reported in trade and consumer literature. See also: durability; toughness; cleavage.