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Although there is no official quality grading system for pearls (like the 4Cs for diamonds), there are several characteristics that affect their value, which you should consider before making a purchase:
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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. 
Like
gemstones, pearls have imperfections. These blemishes or marks on the pearl's
surface should be minimal. 
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. 
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. 
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 and Tahitian pearls. 
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. 
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. 
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. 
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Although there is no official quality grading system for pearls (like the 4Cs for diamonds), there are several characteristics that affect their value, which you should consider before making a purchase:
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