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 accururately 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.

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 (see chart to left).