This is pure knowledge.
Natural pearls are so rare to find in nature that most pearls sold today are cultured. To create a cultured pearl, a tiny bead is implanted into the oyster and gradually over time the oyster coats the bead in many layers of natural minerals and proteins. These layers are referred to as nacre (Nay-Ker.) It is the nacre that gives pearls their beautiful luster and color.
There is a variety of Freshwater, South Sea, and Tahitian cultured pearls in the market.
While industry wide there is no standardized grading for pearls, distributors usually take size, shape, luster, color, surface marking as the indexes.
The general color of a pearl is also called the body color. Typical pearl colors are white, cream, yellow, pink, silver, or black. A pearl can also have a hint of secondary color, or overtone, which is seen when light reflects off the pearl surface. For example, a pearl strand may appear white, but when examined more closely, a pink overtone may become apparent.
Pearls produce an intense, deep shine called luster. This effect is created when light reflects off the many layers of tiny calcium carbonate crystals that compose the pearl. This substance is called nacre. When selecting a pearl, consider that the larger the pearl, the more nacre it has, so it will also exhibit even more luster. Compare a 5mm Freshwater cultured pearl with a 10mm South Sea cultured pearl and the difference in the amount of nacre is obvious. The difference in luster is as clearly visible as the difference in the pearl sizes.
The highest quality, rarest pearl shape is round. Shapes that are not spherical or even symmetrical are considered lower quality. Akoya, Tahitian, and South Sea pearls found in jewelry have a tendency to be the roundest, while Freshwater pearls can be oval or slightly off-round.
As a mollusk creates a pearl, the layers of nacre do not always adhere smoothly. Sometimes spots and bubbles can appear in the layering process. Pearls with the smoothest surfaces are the highest-quality, most sought-after pearls.
The size of the pearl greatly depends on the type of pearl. Freshwater pearls range in size from about 3.0-7.0mm, Akoya pearls range from about 6.0-8.5mm, and South Sea and Tahitian pearls can reach sizes as large as 13mm.
When cared for properly, pearls can last a lifetime. The best way to care for pearls is to wear them often as the body’s natural oils keep pearls lustrous. However, it’s important to keep them away from household chemicals including perfume, makeup and hairspray. Chemicals found in these common products can dull the luster of your pearls. It is recommended that you put your pearls on last when getting ready and make them the first thing you take off when you come home. Before putting your pearls away, wipe them with a soft cloth and store them separate from other jewelry to avoid scratching their tender surfaces.