There are a number of factors or qualities affecting beauty and value of pearls. Here is a short description of What to look for in a pearl:
#6578 natural pearl and diamond brooch.
Size: All other factors being equal, size is the most important of the qualities we look for in a pearl. Bigger is better.
Color: Today, there is a wealth of colors to choose from. Make sure they are natural and not dyed. Sometimes, only a lab can tell the difference. Body-tone may be grey or white while the overtones may be pink, green or blue. Take your pick. Some colors are seen as more desirable than others. For example, the deep gold of South Sea Pearls and "Peacock" Tahitians are highly sought after and command top prices.
Shape: In general we desire a perfectly round pearl. Actually, that is exceedingly rare. Pearls can be round, off round, circled, or baroque in shape. Today, the greatest demand is for baroque shaped South Sea pearls.
#7176 pearls of different colors. 
Luster: This is the shiny quality of a pearl. It may be metallic as in an Akoya, or it may be softer as in a South Sea pearl.
#7455 pearl necklace.
Nacre: this is what makes up the pearl. Layer upon layer of nacre forms around the shell bead. The thicker the nacre, the better the quality of the pearl. If you look at the thread hole of your pearl, you might be able to see where the nacre ends and the bead starts.
Surface: Since pearls are organically formed, it is natural that their surfaces, like ours are not entirely smooth and homogenous. Look for pitting, blemishes, other discolorations or uneven color, smoothness or bumps.
Treatment: All cultured pearls are washed, dried and often buffed after harvesting. However, some pearls are further treated to enhance their appearance and make them more marketable. Altering the color by bleach or colored dye is amongst the techniques used to improve surface appearance and marketability of cultured pearls. In some cases, the treatment is obvious such as neon or radiant colors. Often, only a qualified laboratory can tell whether a pearl's appearance is natural or artificially produced.
#5841 Antique French Natural Pearl Necklace.
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