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Imitation or Not
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Other Tests

X-radiograph Test: An x-ray photo called an x-radiograph is taken of the pearls. Imitations are opaque to x-rays making them look solid white on the negative and solid black on the positive print made of it. Cultured and natural pearls are semitransparent to x-rays and usually look grayish.

Since imitations pearls can be positively identified with other tests, x-raying them is usually a waste of money. There is, however, a major advantage to the x-ray test. It's quicker to x-ray an entire strand at once than to test each pearl in it individually.

Refractometer Test: The "pearl" is placed on a refractometer (an instrument that measures a gem's refractive index-the degree to which light is bent as it passes through the gem). A pearl will generally have a low reading of 1.530 and a high one of 1.685. The numerical difference between these two readings is .155 and is called its birefringence. Pearls have an unusually high birefringence compared to other gems. This causes a blinking and pink effect when their refractive index is read through a rotating Polaroid filter. The GIA Pearls Course states that the presence of this "birefringent blink" is proof a pearl is not an imitation.

The refractive index of some imitations can also prove they are not cultured or natural pearls. For example, the Majorica imitation pearls the GIA tested for their Fall 1990 article in Gems and Gemology had a refractive index of 1.48, which was a conclusive means of identification. Distinguishing between imitations and pearls is not hard. Even lay people can learn how to detect imitations with a loupe, but they do need practice. What's hard is to distinguish cultured pearls from those that are natural.
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