It is also the birthstone for the sun signs of gemini and cancer. See thebirthstone tables for additional references to this stone. Fresh water pearls are given on the 1st wedding anniversary. Pearls are also given on the 3rd, 12th and 30th anniversaries. An organic gem, pearls are formed inside mollusks such as oysters and mussels. They are formed when an irritant such as a tiny stone or bit of sand gets inside the mollusks shell. A lustrous substance, called nacre, is secreted around the object to protect the soft internal surface of the mollusk. As layer upon layer of nacre coats the irritant, a pearl is formed. Light that is reflected from these overlapping layers produces a characteristic iridescent luster. This process of building a solid pearl can take up to seven or eight years. The most valuable pearls are perfectly symmetrical, relatively large and naturally produced. They have a shimmering iridescence which is called orient luster. The principal oyster beds lay in the persian gulf, along the coasts of india and sri lanka, and in the red sea. Chinese pearls come mainly from freshwater rivers and ponds, whereas japanese pearls are found near the coast in salt water. There are many types of pearls natural pearls (made without human interference), cultured pearls (made when a foreign substance is intentionally inserted into a living oyster. This method was first used in 1893),