GemNotes

GIA Study Method Is Flawed, Expert Says

The Gemological Institute of America’s recently published study on diamond proportion and brilliance prompted many to question its conclusions, contending that its critique of current diamond-grading systems is premature without a consideration of fire or scintillation. Now, an accomplished researcher has criticized the very scientific model on which the GIA study authors based their conclusions.

In a letter to the editor in the Winter 1998 issue of GIA’s Gems & Gemology journal, Martin Haske says GIA’s use of three-dimensional hemispherical lighting represents a “fatal flaw” in the study. Haske, who designs and produces high-tech gemological equipment for jewelers and gem laboratories, recognized that the hemisphere, which covered the diamond and stopped at the girdle plane, favored shallow crown angles and possibly larger tables as well. These biases, he says, invalidate the conclusions of the study.

Haske’s critique merits attention because of his technical credentials. Among other accomplishments, the MIT-educated scientist has designed a spectrophotometer that purportedly can determine color grades and treatments in diamonds inexpensively at room temperature. He serves as a consultant for Gemnostic Corp., developer of the Moissketeer 200SD synthetic moissanite tester, a leading-edge thermal conductometer. He has also developed and tested precision guidance and navigation systems for strategic weapons.

A black hole. GIA’s research model was meant to present a uniformly diffuse light, each ray emanating from the hemisphere’s internal surface and striking the crown at every angle of every facet. The amount of light leaving the diamond, or “weighted light return,” depends on the amount of light entering the stone. Says Haske, “They’re basically talking about a diamond which is bezel set [so light can’t enter through the pavilion], viewed in a white room where light can enter the stone from any angle on the crown.”

But the crown facets can’t be struck by light from all angles because the hemisphere doesn’t go below the girdle plane. Haske notes that the girdle plane becomes essentially a black hole. This limits the light that can enter the stone through the crown facets. To make matters worse, the distribution of light entering the crown facets changes as the crown angles change. When lowering the crown angles on the model, you increase the amount and distribution of light from the hemisphere that enters the stone. “If you change the input, you are going to affect the output,” observes Haske.

Notice in Diagram 1 on page 24 that there’s no way light can enter the bezel facets in a full range of angles. In a more accurate model, the light striking the crown facets would do so in the same way the light strikes the table – it would enter at a full range of angles (Diagram 2). Because of this error, as the crown angles become shallower, more light can enter the stone. That results in a greater weighted light return for those figures, which is what the study showed.

What now? Haske feels this “fatal flaw” calls for a retraction of the G&G article and a republication of results with a corrected model, standard procedure when a scientific study is flawed and presents a misleading analysis.

Not so fast, say the GIA researchers. In their published response to Haske in G&G, they question the relevance of his argument without challenging its technical validity. “We can’t reach an informed opinion about Mr. Haske’s work, because no details have been published,” Dr. James Shigley, GIA’s director of research, tells JCK. “Mr. Haske has not demonstrated that he gets different results under different lighting conditions, let alone that we would.”

Shigley concedes that lighting is a nettlesome problem and says GIA intends to explore other lighting geometries in subsequent studies. Meantime, he encourages Haske to publish the results of his own brilliance tests in a peer-reviewed journal. Then, perhaps, it’ll be Haske’s turn to fend off the critics.

Synthetic Blues from Russia

Cap Beesley, president of American Gemological Laboratories in New York, recently examined two synthetic blue rough diamond crystals brought back from a trip to Moscow and Siberia. Beesley notes that “the pace of Russian research for the development and commercialization of laboratory-grown diamonds continues to accelerate.” The Russians are currently producing blue, pink, red, and color-change synthetic diamonds. They’re also trying to create a full rainbow of natural-appearing colors.

In the past, red and pink synthetics received their color post-growth, by irradiation. Now that they’re actually producing red and pink colored crystals, the Russian growers have created a convincing, slightly grayish-blue synthetic diamond.

Beesley’s two blue diamond crystals weigh 0.75 ct. and 1.20 cts. and exhibit the modified octahedral shapes commonly associated with Russian yellow synthetic diamonds. They show short-wave ultraviolet fluorescence with significant phosphorescence – an afterglow. The phosphorescence is pronounced, glowing for hours after only minutes of exposure to UV. The inclusions are typical metallic-like flux.

North American Beauties

“North American Beauties” is a year-long series highlighting gemstones found in the United States, Canada, and Mexico. This month’s installment focuses on pearls from the West Coast, the Mississippi River, and Texas.

American Pearls

Pearls continue to ride a wave of popularity, with natural colors – from the pale pinks to the blacks – leading the way. Unfortunately, the colored pearls aren’t American-grown but come China and the South Pacific. Still, there are some noteworthy developments in the United States.

The North American pearl market contributes to jewelry inventory from three venues: West Coast abalone, freshwater natural and cultured pearls from the Mississippi River and its tributaries, and natural Texas Concho and Colorado River pearls.

Most colorful. Abalone (pronounced ab-a-low-nee) is by far the most colorful of all pearl-producing mollusks, its iridescent colors splayed across both the inside and the outside of the shell. Most important is the inside of the shell, the “mother-of-pearl,” since it dictates the color of the pearl.

Natural abalone pearls are found as far south as Baja California all the way north to the Alaskan coastline. According to Wes Rankin of Pacific Coast Pearls in Petaluma, Calif., American abalone pearls are a rare gem find. Only one quality pearl is found in every 50,000 mussels, although with the recent awareness of their value, more pearls are being uncovered.

Meanwhile, however, the California abalone population is dwindling, making collecting a daunting challenge. Now that the California Fish and Game Department has banned all commercial fishing of abalone, California abalone pearls are gathered only by sport fishermen. With a daily limit of only four shells per diver, this has left Rankin collecting from various California sport fishermen and from commercial harvesters from Mexico and U.S. waters north of California.

Cultured abalone mabé pearls remain in the developing stages, still producing less-colorful pearls than the New Zealand mabés. Don’t look for improvement in the American product anytime soon. The El Niño hurricanes that slammed into Baja last winter reportedly wiped out this year’s entire crop.

Pinks and purples. The more subtle colors of natural and cultured American pearls are coming from the Mississippi River and its tributaries. The round, baroque, and wing shapes of the natural pearls lend themselves to one-of-a-kind pieces. Pink and purple colors dominate the natural hues, although other colors such as green and blue have turned up rarely.

Nadine Nelson, owner of Mississippi Pearl Jewelry Company in Maiden Rock, Wis., says local divers have slowed shell fishing in the face of diminished demand. The Japanese are the biggest purchasers of the shells, and they have all but stopped purchasing. “The Japanese are not doing well,” notes Gina Latendresse, president of American Pearl Co. in Nashville, Tenn., producers of cultured freshwater American pearls. “With disease, pollution, and red tide affecting their production, the nuclei makers have slowed and stopped purchasing the shells.”

Only a few Mississippi harvesters were shelling last summer, hoping to sell their small quantities to the local industry. Complicating matters, the Tennessee Department of Natural Resources has created more regulations to curb over-harvesting. Harvesters are now limited to 50 lbs. of shell per day, with restrictions on the sizes and numbers harvested.

Latendresse notes that just within the last three years the market for natural pearls has increased dramatically, possibly from the resurgence in the antique and estate natural pearl jewelry market. Gathering local natural pearls has become difficult, and quantities have been dwindling over the past 15 years. She still collects from the local fishermen and sometimes trades or purchases natural pearls from international dealers.

American Pearl Co.’s freshwater cultured pearl harvests are still going well, focusing on several different shapes for jewelry designers. “Coin” and bar (rectangular) shapes are the most popular for this purpose. The company’s round pearls compete with the fancy-colored Chinese pearls. About 10% of the harvest yields natural pinks, blues, and golds.

Texas pinks and purples. Texas natural pearl harvests were very low last season as the result of a long drought. With recent rains, there’s hope for a better harvest when the season picks up again in the spring. Colors for the round and semi-round natural Texans appear in beautiful pinks and purples, with some rare greens.

Future stock. Latendresse, who hosts an annual design contest for American-grown pearls, received entries using not only cultured pearls from the American Pearl Co., but also the West Coast abalone and the Mississippi River natural pearls, pearls from Guaymas, Mexico (Baja California), and conch pearls from the Caribbean. (Conch pearls are really round, non-nacreous concretions and technically not pearls.)

Nelson of Mississippi Pearl says experimental keshi farms have been established through the University of Mexico in Guaymas. (Keshi are non-nucleated pearls formed as a byproduct of the culturing process.) The harvesters there are hoping to produce pearls with the iridescence characteristic of pearls from La Paz, directly across the gulf from Guaymas.