How to Grade a Tahitian Black Pearl

The pearls in this necklace have a lower grade, as they aren't perfect spheres.

The the black-lipped oyster Pinctada margaritifera produces black pearls in Tahiti and French Polynesia. The oyster itself is very large, weighing 10 pounds on average, and while the pearls it produces are not exactly black, but instead have hues of charcoal, silver, green and blue, the pearls are called black. Because the pearls come from a natural object they are often imperfect and thus graded on a scale that examines their shape, size, luster, surface quality, thickness and color.

Things You'll Need

• Tabletop
• Magnifying glass

Instructions

1. Roll the pearl across a flat tabletop. If the pearl rolls in a straight line, it is perfectly round, giving it the highest grade for shape. If it veers off in another direction, it's a semi-round pearl, giving it a slightly lower grade. If it can't roll across the table but can spin on one axis, it's a semi-baroque pearl. If it can't roll or spin, it's a baroque pearl, and thus receives the lowest grade for shape.

2. Measure the pearl's diameter in millimeters. Round up to the nearest millimeter, as pearls are not measured in fractions. The larger the pearl, the more valuable.

3. Examine the surface quality of the pearl by holding it under a magnifying glass and turning it over carefully in your hand, looking at every single angle. Quality A pearls are perfectly smooth with zero to one imperfection, such as a microscopic bump. Quality B pearls have some imperfections on less than a third of their surface. Quality C pearls have blemishes on less than two-thirds of their surface. And quality D pearls have moderate to deep blemishes over two-thirds of their surface.

4. Turn the pearl over and assess how much luster is has. If it has very high luster and is very shiny, give it a AAA. If it seems simply shiny, give it a AA. If it is somewhat shiny, give it an A. If the surface is dull or reflects light poorly, give it a B.

5. Evaluate the color. The darker the pearl, the more valuable it is. If the color has a strong peacock green tinge it is extremely valuable and rare.

6. Search for an overtone. Overtones are shades on top of the base color. Common overtones are blue, pink, copper, gold and purple. A pearl without an overtone is worth half as much as a pearl with an overtone.

References

• Pearl Guide: Black Tahitian Pearls
• Black Moon Pearls: Grading

Resources

• Seashell World: Black Pearls