The Mohs scale, developed in 1812, ranks ten minerals from 1 (talc) to 10 (diamond) according to their relative ability to scratch one another. It is not linear—the difference between 9 (corundum) and 10 (diamond) is far greater than between 1 and 2. Most gemstones rate between 5 and 9 on this scale.
Diamond's extreme hardness results from its tightly bonded cubic crystal lattice of carbon atoms. Corundum (ruby and sapphire) scores 9 due to strong aluminum-oxygen bonds. Quartz family gems (amethyst, citrine) rate 7, while softer gems like opal (5.5–6.5) or turquoise (5–6) require protective settings.
Gemologists combine Mohs hardness with toughness (resistance to breaking) and cleavage (tendency to split along planes) for durability assessments. A stone scoring 7 or higher generally withstands everyday wear in jewelry, but gems with perfect cleavage, such as topaz or diamond, need careful orientation during cutting to avoid damage.