Pearls!

Pearls are one of my favorite natural beads for jewelry making, and are most often classified as gemstones. Pearls are created when a tiny grain of sand or other irritant finds its way into an oyster or other hard-shelled mollusk. In order to protect itself, the mollusk begins coating the irritant with layers of luminescent nacre. Eventually, the pearl becomes large enough that it can be harvested and removed from the inside of the shell. Pearls are classified as semi-precious gemstones in jewelry, and some of the finest examples of natural freshwater pearls can cost as much as precious gemstones such as emeralds, rubies, and sapphires! The largest known naturally-occurring pearl was discovered in the Philippines in 1934 in a clam, and weighed in at an astounding fourteen pounds! But because it was from a clam, and not a mollusk that secretes nacre, it didn't have the luster of a gem-quality pearl.

(When Is a Gemstone Not a Gemstone? 06-18-2012 by Jennifer VanBenschoten