On the hardness scale, the jump to diamond is enormous
Diamond sits at 10 on the Mohs scale, but that single step from the runner-up hides a fourfold leap in real hardness.
Diamond is the hardest naturally occurring substance on Earth, anchoring the top of the Mohs hardness scale at 10. Its secret is geometry: every carbon atom is bonded to four neighbours in a rigid, three-dimensional cage, with no weak direction for a scratch to exploit.
The catch is that the Mohs scale is ordinal, not linear — the numbers only rank minerals, they don’t measure how far apart they are. So while corundum (sapphire and ruby) is only twice as hard as topaz, diamond at 10 is about four times harder than corundum at 9 in absolute terms.
One number on the scale, four times the hardness.
Put differently, the gap between corundum and diamond in real hardness exceeds the entire span from the softest mineral, talc, all the way up to corundum. That last step on the scale is by far the biggest.
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