A gem is evaluated mainly for its beauty and perfection. The characteristics that lend aesthetic value to a gem are its color, an unusual optical phenomenon, an incrustation like a fossil, it rarity, and sometimes the peculiar shape of the crystal.
There are four precious stones: diamonds (clear), emeralds (green), rubies (red) and sapphires (blue).
Common stones include amethyst (purple), citrine (yellow to orange), garnet (dark red), and aquamarine (pale blue). Pearls and opals, which are milky white in color, are also considered semi-precious stones.