Pearls are formed when a piece of sand or other irritant finds its way into an oyster. The oyster begins to secrete a smooth hard crystalline substance called nacre around the irritant to protect itself. As long as the irritant remains, the oyster will continue to produce this nacre, layer upon layer. It will take several years to produce a 3 to 4mm pearl.
The most common colors of pearls are silver/ white, rose, cream, gold, and blue/grey. Color choice is mostly up to personal preference. More unusual colors are available, such as blue, purple, black, and green; these colors may be caused by different minerals and nutrients present in the water. Tahiti was the original place for black pearls due to minerals present in that area. Pearls may also be color enhanced. Some believe pearls are for wisdom and that they may help one with inspirational thought.