Hardness is measured using the Mohs Scale. This scale of mineral hardness was created in 1812 by German mineralogist, Frederich Mohs. A gemstone’s hardness value indicates how resistant to scratching it is. The grade is on a scale from 1 to 10, with 1 being very soft and 10 being very hard. Higher number stones can scratch any lower number stone. For example, a gemstone with a hardness of 9 can scratch any gemstone with a hardness of 8 or less. Diamonds are rated a 10, and therefore the hardness and most resistant to scratching of all gemstones. This is why they are so often chosen for engagement rings.
As you can see from the chart below, there is really nothing in the way of everyday objects that compares to gemstones with a hardness over 7. In addition to rarity, hardness makes many gemstones more valuable. Rubies, Sapphires & Diamonds are at the top of the scale, and all are considered precious stones. With the exception of Emerald, gems lower on the hardness scale are considered semi-precious stones. Due to their rarity, especially when found as transparent and without inclusions, Emeralds are considered precious stones.