September 1999

Gemstones & Pearls: Gemology

Circular Logic

The cause of uniform bands around some pearls remains a mystery

More oddly shaped pearls than ever have made their way into the market, including an interesting type with concentric circles seemingly etched into their bodies. Once discarded as an unwanted byproduct of pearl culturing, these so-called circle pearls have established their own loyal following. They can be vastly cheaper than rounder, smoother pearls, and they get double-takes because each one is distinct. What are the oysters that give birth to these pearls doing?

Spinning

The oysters might be trying to rid themselves of unwanted intrusions resulting from pearl culturing. "No one knows 100%," says Fred Ward, author of Pearls (Gem Book Publishers, Bethesda, MD). "If the pearls are stuck in one position, it's possible they may be spun in one direction by the oyster's muscles."

Karen Hurwitt, a senior gemologist at the Gemological Institute of America, offers another opinion. "Lots of factors could play a role," she says. "You must start from the concept the oyster is alive and is being induced with mantle tissue that is meant to lie flat. [Mantle tissue is deposited by pearl culturers through an incision made in the oyster tissue.] Irregularities in the mantle graft could cause the oyster to react in a way that causes unusual formations during the pearl's growth."

The oyster secretes a coating called nacre to envelop the inserted tissue and smooth any rough surfaces. In its attempt to rid itself of the intrusion, it spins the growing pearl in its tissue.
"Not all pearl-growing areas harvest pearls showing the growth rings," says Ward. He points out that circle pearls occur much more in natural black cultured Tahitian pearls than in other South Sea pearls, for example. They also are seen quite commonly in cultured Chinese mantle section freshwater pearls.

"The development of these rings may have something to do with the fact Tahitian oysters, after being nucleated, have a hole drilled in the shell and are hung on a rack in the water. Hanging in place, the pearls tend to spin in only one direction," he says. "Conversely, other South Sea oysters are in baskets that are rotated and flipped so the oyster is not always in the same position."

John Koivula, GIA's chief gemologist, says drag lines can sometimes be seen radiating from a blemish on the pearl's surface, supporting the view at least some pearls spin in one direction during their growth. Ward cautions there's no conclusive evidence to support any one theory.

Marketing Points

The marketing of circle pearls (which have been dubbed "circles of love" in a promotion campaign for Tahitian pearls) is moving into higher gear, thanks to record production worldwide. If you're considering them for your store, remember these selling points:

• Though cultured, circle pearls have a natural look because of irregularities.
• They are available in larger sizes, up to 12mm.
• Prices – even for larger pearls – are reasonable. They sell for just 10%-25% of the cost of comparably sized, rounder, smoother pearls.
• Each pearl is unique and, therefore, a conversation piece.
• In natural black cultured pearls, rich iridescent colors can occur around the ring structure.

  – by Robert Weldon, G.G.