Minerals, including gemstones, are rated and ranked according to their hardness. While hardness is generally associated with durability, the ability to resist breakage is better described as toughness. When referring to gemstones, hardness more accurately means the stone's ability to resist abrasion.

Regardless of which scale is used, the diamond is considered the hardest substance known to man. The most common measure of a gemstone’s degree of hardness is based on the Mohs Scale. Devised by German mineralogist Friedrich Mohs, the Mohs Scale grades minerals on a comparative scale from 1 (very soft) to 10 (very hard).

What the scale means is that a mineral of a given hardness rating will scratch other minerals of the same rating, as well as any minerals of a lower hardness rating. For example, rubies and sapphires, which are composed of the mineral corundum and have a Mohs rating of 9, will scratch each other, as well as topaz (rating 8) and quartz (rating 7). But they will not scratch diamonds, which are rated 10.

Mohs Scale of Hardness

1	Talc	7	Quartz (Amethyst)
2	Gympsum	6.5 - 7.5	Garnet
3	Calcite	7.5	Beryl (Emerald)
4	Flourite	8	Topaz
5	Apatite	9	Corundum (Ruby and Sapphire)
6	Feldspar	10	Diamond

What the scale does not make obvious is that the numeric values assigned to each interval of hardness are not equal. Some stones are disproportionately harder than others:

Gemstone	Mohs Rating	Results from Grinding Test
Diamond	10	Approximately 90 times harder than corundum
Ruby/Sapphire (corundum)	9	Approximately 5 times harder than topaz
Topaz	8	Little difference between topaz and quartz
Emerald	7.5
Quartz	7