Most coloured gems are available in a wide range of sizes, but for some the selection is limited. Gems are generally sold by weight, using the carat, which is one-fifth of a gram. Some gem varieties are more dense than others: a one carat emerald will be noticeably larger in size than a one-carat sapphire or ruby. In general, the larger the gem, the more expensive it will be per carat.

For the classic gems like ruby, emerald, and sapphire, prices per carat can increase dramatically as sizes increase. For example, since large ruby is very rare, a three-carat ruby might be three times the price per carat of a one-carat ruby, or nine times the total price.

Some gems are much more available in large sizes so the price doesn't rise as much as the size goes up. Amethyst, Citrine, Aquamarine, Tanzanite, and Tourmaline might have similar prices per carat for a one-carat and a ten-carat stone; although the ten-carat stone will, of course, still be ten times the price.