THE PREZ SEZ: On Cutting Benitoite
By Nancy Attaway
Of the gem minerals suitable for jewelry, a mineral named benitoite is one of the rarest. It occurs as a gem only in California, and then, only from one very small deposit. With a high refractive index and a dispersion greater than diamond, benitoite reflects and refracts light brilliantly when properly faceted. The deep blue of benitoite compares to that of the finest sapphire. However, a birefringence five times that of quartz and a distinct dichroism makes it easily distinguishable from sapphire.
The discovery of benitoite was made in 1906 or 1907 when a prospector, James M. Couch, staked claims in the name of R. W. Dallas. The claims were located in the area of the Diablo Mountains of San Benito County. Prospector, Couch, was searching for the mercury ore cinnabar or any of the commercial copper ores when he came across a particular hillside, where blue triangular crystals were weathering out of white natrolite veins in serpentine rock. The blue crystals were at first thought to be some peculiar form of obsidian. A parcel of them were sent to a San Francisco lapidary, who faceted many of them. The cutter noted that these strange blue stones were softer than sapphire.
George G. Eacrpt, then the representative of the gem department of Shreve & Company of San Francisco, received on memorandum several of the first stones cut by the lapidary, together with the his notes on their hardness. The cutter's second guess was that they might be spinel, but Eacret's examination revealed that the mystery gems possessed definite dichroic and birefringent characteristics not found in spinel. Strange, indeed.
Dr. George Louderback was, at that time, a famous professor of geology who taught classes at the University of California in Berkeley. Well aware of Louderback's expertise in mineralogy; Eacret sent several of the cut gems to the professor in hopes that he might identify them. Dr. Louderback conducted a thorough series of tests and determined they belonged to a new gem mineral species. He named the new mineral benitoite in reference to the California county, where it was found.
Louderback went on to describe benitoite in great detail in a scientific paper published in 1909. Entitled "Benitoite - Its Paragenesis and Mode of Occurrence" (Bulletin of the Department of Geology; University of California), this paper is considered to be a classic treatise of mineralogy. It is a very comprehensive survey of the morphology and crystallography of this new gem species.
Dr. Louderback also noted that benitoite was the only known representative of the ditrigonal dipyramidal class in the hexagonal crystal system. Remarkably, this particular class of crystal had been predicted in 1830 by the mathematical reasoning of Johann Friedrick Christian Hessel. In a German publication of that date, Hessel correctly predicted the existence and symmetry characteristics of all thirty-two crystal classes. At that time, no representative of the ditrigonal dipyramidal class was known in any substance.
The benitoite deposit, discovered by dames M. Couch, later became known as the Dallas Benitoite Mine after R. W Dallas, who had paid Couch to prospect for him. Lying just to the south of Santa Rita Peak in San Benito County, the mine is at an elevation of 4,000 feet. More precisely, the area is about 25 miles northwest of Coalinga in the New Indria Quadrangle and near Rush Creek, a headwater stream of the San Bonito River. The mine was owned by descendants of R. W. Dallas until 1987 but was not extensively developed.
In 1967, Bill Forrest of Fresno, California and Buzz Gray of Missoula, Montana leased the property. In 1987, they bought it outright. Bill Forrest describes the property as a hillside with exposed veins of white natrolite. Tunnels have been dug to follow the natrolite which contains the blue, triangular crystals of benitoite. Thin white lavers of the natrolite usually coat the benitoite. Bill told me that a 4 to 1 water dilution of sulfuric acid is used to dissolve the natrolite and expose the blue gem crystals. In the case of specimen-grade benitoite, part of the natrolite is left in place to better contrast the crystals against the host rock. I purchased a small benitoite specimen from Forrest during the February 1996 Tucson Gem and Mineral Show. It shows the white natrolite over the gray host rock with several protruding blue crystals of benitoite. My husband, Steve, carefully removed one crystal for me to facet.
In their book, "Faceting for Amateurs", Glenn and Martha Vargas caution that benitoite can be brittle. They recommend cutting using only fine-grit laps. They also recommend using topaz cutting angles for best color and sparkle. Polishing, they say, proceeds readily with either Linde A or cerium oxide on appropriate laps.
I used a 600-grit Dyna Lap to shape the rough crystal to a round form and continued with the pattern for a twelvesided round brilliant stone. I also noted a tendency toward brittleness and an apparent cutting hardness variation in one direction. Polishing did proceed quickly using a cerium oxide charged plastic Dyna Lap after the 600-grit cutting lap.
In the cutting phase, it was necessary to remove the black neptunite
crusts that partially coated the benitoite crystal. Neptunite frequently
occurs with benitoite in the matrix. Casually viewed, the black crystals
resemble those of black tourmaline.
Even though my crystal of benitoite had many inclusions, I certainly was taken with the lovely, deep cornflower blue hue that intensified with every polished facet. In my mind, I tried to imagine the beauty of a clean piece of benitoite cut into a round brilliant or a square barion. With such clarity and such intense blue color, a benitoite of my imagined quality would be absolutely dazzling.
Off the dop and in my hand, the round Flasher cut benitoite sparkled
like a fine blue sapphire, perhaps reminiscent of a Yogo. My 4 1/2 mm round
Flasher cut benitoite weighs 0.41 carats. I look forward to facetting another
one. Hopefully, the next one will have better clarity, fewer inclusions.
I plan to visit with Bill Forrest again come next February and the Tucson
shows. You can be sure I'll check the possibility of purchasing yet another