A pearl is formed within the soft tissue of an oyster or other shelled mollusk. It is composed of Calcium Carbonate and can be formed when a grain of sand or other microscopic particle infiltrates the oyster and the oyster produces the coating to protect itself.
Pearls are typically known as white, but pearls have been known to be cream, pink, silver, brown, green blue, black and yellow - and pearls rate 2.5-4.5 on the Mohs Hardness scale.
Pearls can be natural or cultured and the main difference between the natural and cultured is that the cultured pearls have a particle inserted manually and due to the economics involved they normally insert a much larger particle or graft and this means that cultured pearls can be distinguished by x-ray examination - where a cultured pearl will show a solid centre with not many growth rings whereas a natural pearl will have concentric growth rings from the centre.
Examples of pearls are shown below.