Real pearls can be either natural or cultured. Natural pearls are formed when a mollusk secretes a substance called nacre to coat an irritant that makes its way inside the animal's shell. Cultured pearls are formed with a little human help -- an irritant is purposely introduced into the shell of the animal. You usually need an x-ray or an expert to distinguish between natural and cultured pearls.
Pearls differ from other gemstones in that they're not mined from rock, but rather harvested from mollusks, especially oysters. Clams and mussels have been known to produce pearls as well. Basically, a pearl is produced when an irritant such as sand gets caught inside the oyster. To protect itself, the oyster secretes a substance akin to mucus. This substance builds up around the sand and hardens. The end result is a pearl.
Natural or South Sea pearls are those made by mollusks harvested from the ocean. Every now and then, one of the pearls will be a darker shade, either gray or black. These are rare, and as a result, very expensive.
As you can well imagine, it's a long, tedious practice to find oysters that have ingested a piece of sand to produce a pearl. The longer a pearl stays inside an oyster, the larger it becomes. Because of this, quality pearls are considered rare and cost a pretty penny. Fortunately for those who favor this gem, there are now farms in which the irritant is deliberately injected into the mollusk to produce the pearl. These pearls are known as cultured.
When pearls are made without the benefit of a mollusk, they're considered man-made. Man-made pearls are made by using a bit of sand or shell along with a fish-based product called hermage to act as a substitute for the oyster's mucus. Other synthetic pearls include the "glass" pearl, a bead painted with hermage and left to dry and harden. These pearls have little or no value.
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