The value of a pearl is determined by its Lustre, Surface, Shape, color and Size.
Lustre is the most important factor in choosing pearls. The inner glow of the pearl combined with the surface brilliance defines lustre. The higher the lustre, the thicker the nacre or secretion from the oyster and the stronger the glow. Lower quality cultured pearls appear too white, dull or chalky.
The smoothness of the pearl's surface, from clean to heavily blemished, is the next consideration. Cracks or breaks in the nacre are considered damage. Because pearls are grown in an oyster and are organic gems they are almost never flawless. The gem-quality pearl may have minute blemishes when examined very closely, but they are not noticeable at arm's length.
Similarly, it is very rare to find a perfectly round pearl. The rounder the pearl, however, the more valuable it is. Slightly off-round, semi-baroque and baroque pearls are not as valuable as perfectly round pearls, however, they can be lustrous and appealing and have a natural beauty and value of their own.
Cultured pearls range in color from white to black with various mult-colored overtones. The color of the pearl is really the wearer's preference.
Cultured pearls are measured in millimetres. All other factors being equal, the larger the pearl the rarer and more valuable it is.
A cultured pearl is an organic gem. This means that it was produced by a living organism, generally an oyster or a mussel. It is produced by placing a bead inside the oyster. The oyster then coats the bead (with nacre) to produce the pearl.
A shell based pearl is an imitation. It is made by painting a shell bead with pearl paint. Be careful of brand named shell based pearls, these sometimes appear to be cultured pearls, but are only imitation.
A freshwater pearl is a pearl that is cultured in a mussel in a lake, whereas the traditional cultured pearl is cultured in an oyster in the ocean (salt water pearls are called Akoya pearls). Freshwater cultured pearls are less expensive than the Akoya pearls.
Freshwater pearls come in many shapes from round to button shapes, from flat flakes to rice shapes and in many colors. As with the salt water Akoya pearl the round shapes are generally more expensive.