Not all round pearls are round. They are classified as round, slightly off round and off round. Nacre is the lustrous protective coating oysters produce over intruding irritants. Pearls are measured by size, not weight. Uniformity of size is the most important factor in strands.
Pearls have very porous surfaces.
If you see a dark spot, it could be from perfume. Orient is the iridescent play of color deep inside pearls. You will most likely not find this in cultured pearls because very thick nacre is needed to create the effect. All the most important valuing factors for purchasing pearls on one handy sheet. All pearls are tumbled clean upon removal from the shell.