Diamonds may get the credit for being "a girl's best friend," but humanity's love affair with pearls has been going on for 4,000 years — about a millennium longer than we've been digging diamonds out of the dirt.

What's more, pearls are the only "green" natural gemstone; they are a renewable resource and are harvested, rather than mined.

While you may have heard about the basics of diamond buying, few consumers are as educated about the nuances of pearl purchasing.

Before you buy pearls, here's what you should know:

Types and sourcesFour types of pearls are available today.• Freshwater pearls, known for their off-round shapes, fancy colors and affordability, are almost exclusively produced in China. Prices range from as little as $20 for a strand of low-grade freshwater pearls up to several thousands.• First cultured and sold in Japan nearly 100 years ago, akoya pearls are the type most people think of when talking about pearls. They come from China, Japan, Vietnam, Korea and Australia.• Although they're also known as "black pearls," Tahitian pearls actually come in every color of the rainbow. They are prized for their large size and exotic colors. The Tahitian Islands are the main producers, but some production also occurs in the Cook Islands, Fiji, Micronesia and Japan.• The most valuable of all cultured pearls, South Sea pearls are renowned for their large size and satiny luster. A large perfect strand of these white, golden or silver pearls can cost millions of dollars. Australia is the primary producer of the white and silver pearls, while the Philippines and Indonesia produce golden pearls.

How to evaluate qualityLike diamonds, pearls have value indicators: luster, surface, nacre, matching, color and shape.• Luster — When dealers price pearls, the single most important factor in their decision is the luster. Pearls should have a sharp, reflective quality. You should be able to see your reflection.• Surface — The pearls should be free of marks, abrasions, cracks, pits or any other imperfections.• Nacre — The thickness of the nacre — the "coating" of pearl that surrounds the bead that was inserted into the oyster — is important when dealing with saltwater pearls, especially akoya pearls. If the coating is too thin, the pearls will be chalky and will not last more than a few months to a year. If the pearls are chalky, or you can see a bead through the surface, the nacre is too thin.• Matching — Whether you are buying a strand of pearls, a pair of earrings, a bracelet or an earring and necklace set, each pearl in the set should match in size, color and luster.• Color — Another important value factor, color is heavily dependent on preference.• Shape — Pearls come in many different shapes, from wings and potato to rice crispie (or baroque) and round. The more perfectly round a pearl is, the more valuable the strand if all other value factors are equal.