Professional Jeweler Archive: Enigmatic Pearls

September 2000

Gemstones & Pearls: Gemology

Enigmatic Pearls

Solving the mystery of the beautiful, if confusing, cultural freshwater pearls from China

The quest to answer questions of how cultured freshwater pearls from China are formed has brought together the American Gem Trade Association, the Gemological Institute of America and a researcher from Mikimoto Co.

“The vast majority of freshwater cultured pearls from China are tissue-nucleated and not all-nacre bead-nucleated, as has been suggested,” says Kenneth Scarratt, director of the AGTA Lab, New York City. Scarratt, Tom Moses of GIA’s Gem Trade Laboratory and Shigeru Akamatsu, a researcher for Mikimoto in Japan, collaborated on a study of the pearls. Details were published in the Summer 2000 issue of Gems & Gemology, GIA’s quarterly publication. The findings are based on travel to Chinese pearl farms, interviews with witnesses and analyses of more than 40,000 of the pearls.

X-Ray Analysis the Key

The researchers mainly used X-radiography. Scarratt acknowledges industry criticism of X-rays as a diagnostic tool – some say it’s a limited way to detect growth in Chinese freshwater pearls. But the techniques used were different in this study. This fine-tuning included:

  • Using fine-grained film that preserves the detail necessary for examining at 10X or 12X magnification.
  • Taking several exposures. Some features show up in greater relief with darker exposures; others show up better with lighter exposures.
  • Taking X-rays in several directions, allowing for a three-dimensional composite. This is critical because at one angle, structural features such as cavities might appear large and bulky. Viewed at another angle, the feature may seem elongated and thin.

No Conchiolin Layer

Moses says that if all-nacre beads were used for nucleation, X-rays would reveal a concentric conchiolin-rich layer. (Conchiolin is a substance the mollusk secrets to protect itself from foreign intrusions.) “What we found missing in the Chinese freshwater pearls is that concentric ring,” Moses says.

This conchiolin layer is visible in X-rays of cultured pearls containing shell beads. Moses says X-rays penetrate that layer easily, but have a tougher time penetrating the subsequent deposit of nacre. The differing exposures reveal the outline of a conchiolin layer coating the bead, whether it’s a shell bead or an all-nacre bead. None of the thousands of pearls Moses and Scarratt examined revealed that outline.

Scarratt says bead nucleation does occur on an experimental basis. “When beads and mantle tissue are inserted in the mollusks, they expel the bead but not the mantle tissue,” he says. The cause might be a matter of space in the mollusk. Scarratt says researchers have had some success inserting beads in the gonad area of the mollusks (where there is more room to grow), but there’s no economic imperative to culture pearls this way. “If you can successfully cultivate 30 pearls in a single mollusk using tissue nucleation, it doesn’t make economic sense to cultivate one to two pearls by bead nucleation,” he says.

Apple Cores

“While we found no evidence of bead nucleation, there appears to be a cavity in all of the pearls we examined, in some ways similar to that seen in natural pearls,” says Scarratt. Moses says this is significant because tissue nucleation leaves behind a void as the tissue decays.

The size and winding formation of the cavity depends on the pearl and can be determined only by taking cross-section X-rays, says Scarratt. (Cross-sectioning a pearl is a destructive test and should be used only for research.) The cavities can take up a huge space within the pearl, in some cases resembling the core of an apple.

Concentric Layers

Like natural pearls, these cultured freshwater pearls are rarely exactly round. “They are near-round,” says Scarratt. “If bead culturing were taking place, we would see many more round pearls, since beads inserted in a mollusk essentially control the outcome of the shape.”

Nonetheless, Chinese farmers have been producing bigger and more rounded pearls in recent years, a factor Scarratt says relates to the size of the mollusk. Before 1989, the Cristaria plicata species of freshwater mollusks used were quite small; since 1989, however, Chinese farmers have used Hyriopsis cumingi mollusks. “These larger mollusks afford more room for pearls to grow with a rounded shape, as they naturally want to do,” says Scarratt.

Another clue pointing to tissue nucleation is revealed in cross-sections of the cultured freshwater pearls: uniform concentric color bands of nacre. “These are growth features of the pearl’s nacreous layer and relate to changes in temperature, mollusk food supply or migration of the pearl within the shell,” says Scarratt.

If bead nucleation were used for the majority of pearls on the market, cross sections of the pearls would reveal patterns out of sync with the growth of the rest of the pearl, says Scarratt. For example, an elongated baroque freshwater pearl would show uneven elongated concentric layers. If a portion was rounded and used as an all-nacre insert, the resulting layers would not form in the uniform concentric patterns seen in cross-sectioned pearls. “You might be able to achieve one perfectly layered pearl as a coincidence,” he says “but not on the massive scale we see in the market.”

There is confusion about how long it takes for the pearls to grow. “It’s quite likely they too have a seven- or eight-year growth period, not a two-year growth period as has been theorized,” says Scarratt.

Evolution or Revolution?

The industry clearly has a new product and is experiencing growing pains. Scarratt likens it to what happened to the natural pearl market when Mikimoto introduced the first cultured pearls. “The natural market is still there and is probably about the same size. What changed is that the overall pearl market expanded dramatically,” he says. There is little for the pearl industry to fret about, he says. Cultured freshwater pearls are a different product than cultured salt water pearls, and each will find a market.

Cultured freshwater pearls from China have enlarged a market hungry for a quality gem boasting myriad colors and a multitude of sizes and shapes, say researchers.

– by Robert Weldon, G.G.

Tissue-Nucleated Pearl

X-rays of the Chinese freshwater pearls show minute cavities in the centers of the pearls that may radiate out. This feature indicates the pearl was tissue nucleated.

Cross-Sections Would Tell

All-nacre beads fashioned from baroque freshwater pearls would have their own concentric layers distinct from round pearls. This rounded pearl, used as a bead nucleus, would have different nacreous layers than those growing around it. This pattern wasn’t found in the Chinese freshwater pearls studied.

Bead-Nucleated Pearl

A typical X-ray of a bead-nucleated pearl shows a conchiolin-rich layer forms around the bead.

These are more typical of the near-round cultured freshwater pearls from China. They are courtesy of Rafco International Gem Corp., New York City; (800) 697-8277 or (212) 221-3435.
Cultured freshwater pearls from China have intrigued the jewelry industry because of their unique pastel colors, nearly all-nacre composition and relatively reasonable prices. This rare necklace of Chinese pearls is composed of a series of culturing “mishaps” that resulted in a stunning strand courtesy of Mellika Co. Inc., Albuquerque, NM; (505) 898-2888.

Copyright © 2001 by Bond Communications