In 1917, Cartier bought their building in New York with two strands of natural pearls valued at a million dollars. In 1957, the pearls were sold at auction for $157,000. Perhaps one of the main reasons for this drop in price, was the introduction of the cultured pearl, which decreased the demand for natural pearls.

Prices of natural pearls have risen considerably since 1957, but they still don't match those of the early 1900's. Nevertheless, natural pearls are still worth a lot more than cultured pearls. Therefore, it's important to be able to distinguish between them. X-ray tests are generally required to prove a pearl is natural, but they are costly. Other tests can help you determine a pearl is cultured and thereby save you the expense of an x-ray. These tests are listed below. Keep in mind that almost all the pearls produced today are cultured. You are most likely to find natural pearls in antique pieces. (Whole pearls were not cultured before the 1900's.) However, the natural pearls in antique jewelry may have been replaced with cultured ones.

Tests a Layperson Can Do

Drill Hole Test:  Look inside the drill hole with a 10-power magnifier. If you can see a dark dividing line separating the nacre from a pearl bead nucleus, the pearl is cultured. This dark line is conchiolin, the material which binds the nacre to the bead. Natural pearls may show a series of growth lines, which get more yellow or brown towards the center of the pearl. A black deposit at the center of a white pearl can be a sure sign the pearl is natural.

Also note the size of the drill hole. The drill holes of natural pearls are rarely larger than .04 mm (.016 inch). Those of cultured pearls tend to measure .06 mm (.024 inch). Natural pearls are partly valued by weight, so the holes are made as small as possible to minimize weight loss.

Shape Test:  Do the pearls look perfectly round? If so, then it's likely they're cultured. Natural pearls tend to have at least slightly irregular shapes, even though a few are round. This test is only an indication. It is not proof.

Blink Test:  Hold the strand near the front edge of a strong desk lamp. The light should shine through the pearls but not in your eyes. Rotate the strand. If the pearls blink from light to dark as they are turned, this indicates they are cultured and have a thin coating of nacre (imitation pearls with mother-of-pearl shell-bead centers may also blink). The dark areas result when there are dense mother-of-pearl layers on the shell bead which block the light. Cultured pearls with thin nacre may show only one view when rotated. In other words, they don't necessarily blink.

Stripe Test:  As you rotate the pearls with strong light shining through them, look for curved lines and stripes. These are the growth layers of the shell beads. If they are visible, the nacre is very thin and the pearls are cultured. Not all shell bead nuclei show stripes, though. This can be seen in figure 14.7. Keep in mind that imitation pearls with shell-bead centers can also display this banded effect. Natural pearls, however, will not look striped.

Color Test:  Examine the color. Cultured pearls often have a faint greenish tint, unlike natural pearls. Some dealers find that the color of natural pearls has a greater potential for brightness than that of cultured pearls. Color can only suggest a pearl might be cultured. It is not proof.

Matching Test:  Because of their rarity, it's difficult to find natural pearls that match,

Consequently, natural strands do not appear as well matched for color, shape, luster and size as those which are cultured.

Other Tests

X-radiograph Test:  This is the most reliable way to distinguish between natural and cultured pearls. On an
x-radiograph negative, cultured pearls usually show a clear separation between core and nacre. Plus, their core normally looks lighter than the nacre coating. X-rayed natural pearls tend to either look the same tone throughout or get darker in their center. A mantle tissue nucleus will look like a very dark, irregular-shaped void.

The disadvantage of x-ray tests is that it can cost between $100 and $300 to have them done, and there are few gem labs that have the required equipment. Stephen J. Kennedy of the Gem Testing Laboratory of Great Britian provides good photos of x-radiographs in an article entitled "Pearl Identification" in the January-March 1998 issue of the Australian Gemmo­logist. This is also an excellent source of additional information on the identification of natural pearls.

X-ray Fluorescence: This test is used in combination with an x-radiograph to provide the added information of whether the pearl is freshwater or saltwater. Natural saltwater pearls rarely fluoresce to x-rays. Freshwater pearls have a fairly strong yellowish x-ray fluorescence, whereas bead-nucleated salt-water pearls.
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