As natural pearls are so rare to find in nature almost all pearls sold today are cultured. This means that a bead is implanted into the oyster so that over time the oyster will coat the bead in mineral and protein layers. Layers are referred to as nacre, which give pearls their magnificent lustre or shine. The larger the pearl, the more lustre it will show. Most pearls are white, cream, yellow, pink, silver or black however, they can have overtones which reflect under light. The size of a pearl depends on the kind of pearl that it is. Freshwater pearls range in size from about 3.0-13.0mm, cultured pearls range from about 3.0-8.5mm, and South Sea and Tahitian pearls can reach sizes as large as 13mm. The smoother a pearl the more rare and then more valuable it is. The most sought after shape for a pearl is round, the more symmetrical a pearl, the greater its value