Even though cultured pearls are real pearls, there are a couple tests that can determine the difference between cultured pearls and natural pearls. These tests are not necessary unless you are looking at antique pearl jewelry or if a jeweler tells you his pearls are natural.

Drill hole test
Natural pearls are composed entirely of nacre while cultured pearls have nacre surrounding a mother-of-pearl nucleus. This can be seen as a dark brown dividing line when examined through a jeweler’s loupe.

This is not a foolproof test, as some cultured pearls have a thick nacre where the brown line will not be visible. This line can also be bleached away. If you don’t see a brown line, more testing should be done.

Ultraviolet test
Under a regular UV light, natural pearls will emit a yellow-tan color, while cultured pearls emit a milky-blue color.

There are also fiber-optic and X-ray tests that a gemologist can perform which are fairly foolproof.