Mohs Scale.

The Mohs scale is the small ruler sitting behind every "is this stone scratch-resistant?" question. It was proposed in 1812 by the German mineralogist Friedrich Mohs (1773 to 1839), who picked ten reference minerals he had to hand at the imperial mineralogical collection in Vienna and ranked them by which could scratch which. The result is a comparative ordinal scale, not a measurement of any underlying physical property, but it has held up because the test is cheap, the reference minerals are widely available, and the order tracks the hardness of the chemical bonds reasonably well.

The ten reference minerals

Talc sits at 1 (a fingernail will mark it easily, soapstone is mostly talc), gypsum at 2 (a fingernail will just scratch it, selenite and alabaster are gypsum varieties), calcite at 3 (a copper coin will scratch it, the basis of marble and limestone), fluorite at 4 (a steel knife scratches it without effort), apatite at 5 (a steel knife still scratches it, but with more pressure; tooth enamel is a form of apatite), orthoclase feldspar at 6 (it will not be scratched by a steel knife and itself can scratch glass), quartz at 7 (the threshold for everyday durability, scratches glass cleanly), topaz at 8, corundum at 9 (the species that includes ruby and sapphire), and diamond at 10. A few common reference points are useful: a fingernail is around 2.5, a copper coin around 3, a steel knife around 5.5, window glass around 5.5 to 6, and a streak plate around 7.

The scale is comparative, not linear. The jump from 9 to 10 is far larger than the jump from 1 to 2. Modern measurements with a sclerometer or by absolute indentation testing show that diamond is roughly four times harder than corundum in absolute terms, while corundum is only about twice as hard as topaz. This is why diamond cuts everything else, even though it is "only" one step above corundum on Mohs's list.

Why it matters at the bench

A useful rule of thumb: anything below 5 will be scratched by everyday grit, since common street and household dust contains a high percentage of quartz at 7 plus airborne corundum (often less than 1 percent but enough to abrade). This is why softer stones like selenite (2), malachite (3.5 to 4), turquoise (5 to 6), and pearl (2.5 to 4.5) need careful handling, while quartz and harder stones can live in a pocket without much fuss. Daily wear pieces in rings, where the stone takes constant impact and abrasion, are best chosen from 7 and up: quartz, topaz, corundum, diamond, the harder garnets, and tourmaline (7 to 7.5).

A common misconception is that hardness equals toughness. It does not. Hardness measures resistance to scratching. Toughness measures resistance to breaking. Diamond is hard but can be cleaved with a sharp tap on the right plane. Jade, at around 6 to 7, is famously tough because of its fibrous interlocking structure, and survives drops that would shatter a topaz at 8. A second misconception is that the Mohs number is a quality measure. A soft stone is not a lesser stone, just a more vulnerable one. Many of the most cherished gems in the world (opal at 5.5 to 6.5, turquoise at 5 to 6, lapis lazuli at 5 to 6, malachite at 3.5 to 4) sit well below quartz. They simply ask for different settings, gentler care, and a different conversation with the buyer about how to wear them. Hardness is a practical guide, not a verdict.