Pearls are an organic gem, created by a living creature. When a piece of grit or other particle gets inside a mollusc shell, the creature coats the intruder with layers of nacre (or Mother of Pearl, MOP), eventually forming the pearl.
Natural pearls are rare and expensive, and are generally found in antique jewellery. Most of today's pearls are cultured. That is, shells are carefully opened and an object is placed inside to encourage the development of the layers of nacre.
Freshwater pearls grow inside a freshwater mollusc, living in a lake or river, rather than inside a saltwater mollusc.
There are three shapes of pearl; spherical (round), symmetrical (pear-shaped, oval, anything symmetrical except round) and baroque (irregularly shaped). Baroque pearls tend to be the least expensive but are quite unique. Miss Milly jewellery uses both symmetrical and baroque pearls.