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If you were to cut a 7-mm Akoya cultured pearl in half, you would see a large core inside. It would be a bead probably cut from an American mussel shell. The outside of the bead would be encircled with a pearly layer of nacre. If the pearl had been left in the oyster for just six months, the layer would be very thin, too thin to be very durable or lustrous.

Judging Nacre Thickness

If you were to cut a 7-mm Akoya cultured pearl in half, you would see a large core inside. It would be a bead probably cut from an American mussel shell. The outside of the bead would be encircled with a pearly layer of nacre. If the pearl had been left in the oyster for just six months, the layer would be very thin, too thin to be very durable or lustrous.

Before about 1960, Japanese Akoya pearl farmers left the pearls in the oyster for at least two and a half years. Mikimoto left his in for over three years for maximum nacre thickness. Then many farmers dropped the time to one and a half years. Around 1979, pearl harvesting started to be done just after six to eight months. The result-a lot of inexpensive, low-quality pearls on the market. And they are still out there, being offered at rock-bottom prices. The buyers end up with shell beads and hardly any pearl. Fortunately, better pearls with thicker nacre are also available, but rarely as thick as those cultured before the 1960's. The goal of this section is to help you determine if the nacre thickness of the pearls you look at is acceptable or not.

The GIA (Gemological Institute of America) defined 5 levels of nacre thickness for Akoya cultured pearls in their pearl grading course which was copyrighted in 1990:

Very thick                At least 0.5 mm on all the pearls of the strand
Thick>                     At least 0.5 mm on most pearls of the strand
Medium                   0.35-0.5 mm on most pearls
Thin                         0.25-0.35 mm on most pearls
Very thin                  0.25 mm or less on most pearls

The GIA's latest categorization of nacre quality (2001) has just three classifications:

Acceptable—Nucleus not noticeable, no chalky appearance
Nucleus visible—The cultured pearl shows evidence of its bead nucleus through the nacre
Chalky appearance—The cultured pearl has a dull appearance
Pearl dealers don't need to measure the nacre to determine if it's thin or very thin. They know just by looking at the pearls. Some clues are:

• The pearls usually have a low or very low luster and may look milky. Some thin-coated pearls, however, may show a decent medium luster.

• The nacre coating has cracks.

• Areas are visible where the nacre has peeled off.

• Layers of the shell beads are slightly visible when the pearls are suspended and light shines through them. These layers look like curved lines, stripes or wood grain. Usually the thinner the nacre, the easier it is to see the lines. If you can't see any shell layers, this does not mean that the nacre is thick. There are lots of thinly-coated pearls that don't show these layers. However, if you can see them, the nacre is probably too thin.

• As the beads are rolled, some may look light and then dark as the light shines through them. This is because the shell beads may have mother-of-pearl layers that block the light.
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