The tradition of wearing Pearls on your wedding day stems from the Ancient Greeks, it is one of the worlds oldest gemstones taken from the latin meaning "unique". The Greeks believed that pearls promoted marital harmony and symbolised happiness, chastity and purity.
In the Louvre in Paris there is a fragment of the oldest known Pearl which was found in the coffin of a Persian princess who died in 520BC.
Almost all Pearls sold today are cultured Pearls, the way they are created is slightly stomach turning for such a beautiful end result. A slight opening is made in a live mollusc and a tiny foreign object or even a small Pearl is inserted, this understandably annoys the mollusc and to protect itself it covers the object with a substance called nacre.
Once nucleation has taken place it is then attatched to a wire frame and suspended from a float into water. Ten or more Pearls can be cultured from a single muscle with various ranging colours.
Natural Pearls are an accident of nature and are formed randomly when a parasite becomes lodged and it covers the parasite in nacre, this can take several years (it seems to me being an oyster is not much fun, if they are not nacre-ing Pearls they are being swallowed whole in restaurants)
Why do some superstitions say Pearls can cause tears? Once the nobility discovered Pearls they all wanted one causing several small European houses to bankrupt themselves therefore leaving them in tears.