High quality pearl has been regarded as one of precious objects of beauty for many centuries, on par with other gemstones. Technically, a pearl is produced inside a living shelled molusc, within the soft tissue or the mantle. Pearl is made of calcium carbonate in minute crystalline form, just like the shell of a calm, in concentric layers. An ideal pearl would be perfectly round and smooth but there are many other shapes of pears, called baroque pearls. Because pearls are made primarily of calcium carbonate, they can be dissolved in vinegar. Calcium carbonate is susceptible to even a weak acid solution because the crystals of calcium carbonate react with the acetic acid in the vinegar to form calcium acetate and carbon dioxide.
Natural pearls that occur spontaneously in the wild are the most valuable but at same time are very rare. Pearls that currently available on the market are mostly cultured or farmed from pearl oysters and freshwater mussels. Imitation pearls are also widely produced as inexpensive jewelry although the quality is much lower than the natural ones.
Artificial pearls have poor iridescence and are easily distinguished from natural ones.
The quality of pearls, both natural and cultivated ones, depends on its being nacreous and iridescent as is the interior of the shell that produces them. While pearls are mostly cultivated and harvested to make jewelry, they have been also stitched onto lavish clothing as well as crushed and used in cosmetics, medicines and in paint mixtures.