Pearls

Color:
Although pearls can come in a wide range of shades, the major color classifications are white, pink, silver, cream, gold and black. Finer quality pearls have an overtone, which usually appears toward the outside edge of the pearl. This can be rose, green or blue. Regardless of the color or shading, you should look for pearls with a deep, rich color that seems to come from within the gem.
Shape:
Pearls that are perfectly round are extremely rare, so they are naturally the most valuable. While most pearls appear to be round, only about 1% are actually perfect spheres. If you want to know if a strand consists of perfectly round pearls, just roll it on a flat surface, like a counter or tabletop. If it rolls smoothly and evenly, the pearls are round.
Luster:
The surface shine that gives pearls their unique glow is known as its luster. The sharper the reflection of light on a pearl's surface, the more luster it has.
Surface Blemishes:
Like gemstones, pearls have imperfections. These blemishes or marks on the pearl's surface should be minimal.
Size:
All other factors being equal, the size of a pearl is the most important factor in determining its value. However, a larger pearl is not necessarily more valuable if it lacks nacre thickness. A pearl that does not have many layers of nacre will crack and discolor over time, diminishing its value. Only an expert can determine the true thickness of a pearl’s nacre. Pearls are available in a wide range of diameters (measured in millimeters), from 6.5mm up to 15mm or more in the case of Australian or Tahitian pearls.
Orient:
The many layers of nacre on a pearl produce rainbow-like colors that appear to move on the pearl’s surface. This motion effect is very rare and only appears in a small percentage of round pearls.
Uniformity:
It is critical that the pearls in any strand or piece of jewelry be evenly matched in terms of their size, shape, color, luster, etc. Visible variations of these qualities not only affect the piece’s beauty, but its value.
Availability:
A pearl’s value is also based on its availability. Saltwater cultured pearls generally command higher prices than freshwater cultured pearls, since a saltwater oyster can only produce one or two pearls at a time and a freshwater mussel can produce as many as 50 pearls at a time.