Pearls belong to the class of organic gemstones. They are products of bivalve mollusks (mainly oysters and mussels) built up of nacre (mother-of-pearl), which is mainly calcium carbonate in the form of aragonite crystals, and another type of organic horn substance known as conchioli.
Although pearls are not especially hard, only about 2.5 to 4.5 on the Mohs scale, they are extraordinarily compact and very difficult to crush. The colors of pearls can vary depending on the type of mollusk and the water they were formed in. Natural pearls are incredibly rare and extremely valuable (occurring without human intervention), so most pearls available today are actually cultured pearls. Cultured pearls are available as both saltwater and freshwater pearl.
It's pretty great having a whole lot of different gemstones to choose from. When doing crafts and jewelry making, it's great to have all kinds of options, and it's amazing how much cheaper it is to just purchase un-set gemstones and make the setting yourself, as opposed to buying fully-made jewelry already. It's also extra special since you made it yourself.