Diamond is the hardest known naturally occurring stuff, scoring 10 on the relative Mohs scale of mineral hardness and having an total hardness value of between 167 and 231 gigapascals in a number of tests. Diamond's hardness has been known ever since antiquity, and is the origin of its name. Nevertheless, aggregated diamond nanorods, an allotrope of carbon first synthesized in 2005, are now known to be even harder than diamond.

The hardness of diamonds also contributes to its suitability as a gemstone. Because it can just be scratched by other diamonds, it maintains its polish exceptionally well, keeping its luster over extensive periods of time. Unalike numerous other gems, it is well-suited to everyday wear because of its resistance to scratching perhaps contributing to its reputation as the preferred gem in an engagement ring or wedding ring, which are many times worn every day.

Unlike hardness, which sole denotes resistance to scratching, diamond's toughness is only fair to good. Toughness relates to a material's capacity to resist breakage from strong impact. As with any material, the macroscopic geometry of a diamond contributes to its resistance to breakage. Diamonds cut into certain particular shapes are therefore much more prone to breakage than some others.

Diamonds appear in a variety of transparent hues colorless, white, steel, blue, yellow, orange, red, green, pink, brown or colored black. Diamonds with a detectable hue to them are known as colored diamonds. Colored diamonds have in it impurities or structural defects that cause the coloration, while pure or almost pure diamonds are transparent and colorless.

The carat weight measures the mass of a diamond. One carat is defined as exactly 200 milligrams (about 0.007 ounce). The point unit equal to one one-hundredth of a carat (0.01 carat, or 2 mg) is commonly worn for diamonds of less than one carat.

All else being equal, the value of a diamond increases exponentially in relation to carat weight, because larger diamonds are both rarer and much more desirable for use as gemstones.

The price per carat does not grow smoothly with increasing size. Instead, there are sharp jumps around milestone carat weights, as demand is much higher for diamonds weighing just more than a milestone than for those weighing just under. As an example, a 0.95 carat diamond may have a significantly lower price per carat than a comparable 1.05 carat diamond, because of differences in demand.