Fall 1997 Gems & Gemology

EMS EMOLOGY GFALL 1997&G VOLUME 33 NO. 3 TABLE OF CONTENTS EDITORIAL 165 Symposium 1999: Meeting the Millennium William E. Boyajian FEATURE ARTICLES 166 Benitoite from the New Idria District, San Benito County, California Brendan M. Laurs, William R. Rohtert, and Michael Gray pg. 185 188 Tairus Hydrothermal Synthetic Sapphires Doped with Nickel and Chromium Victor G. Thomas, Rudolf I. Mashkovtsev, Sergey Z. Smirnov, and Vadim S. Maltsev 204 Multicolored Bismuth-Bearing Tourmaline from Lundazi, Zambia Mary L. Johnson, Cheryl Y. Wentzell, and Shane Elen EGULAR EATURES pg. 189 R F 212 Gem Trade Lab Notes 220 Gem News 231 1997 Challenge Winners 233 Book Reviews 235 Gemological Abstracts ABOUT THE COVER: Commercial quantities of gem-quality benitoite are found at only one location in the world, the Benitoite Gem mine, in San Benito County, pg. 205 California. This historic locality continues to produce fine-quality gemstones and mineral specimens, and an extension of the deposit was found in the spring of 1997. The feature article in this issue looks at the history of the Benitoite Gem mine, the geology of the pg. 214 New Idria district and current mining there, and the gemological properties of this dis- tinctive material. The benitoite and diamond necklace illustrated here is shown with the largest faceted benitoite to date, a 15.42 carat stone from the Benitoite Gem mine. An artistic rendering, by GIA designer Lainie Mann, features this stone mounted in a pen- dant. Jewelry and gemstone from the collection of Michael M. Scott. Photo © Harold & Erica Van Pelt––Photographers, Los Angeles, California. Color separations for Gems & Gemology are by Pacific Color, Carlsbad, CA. Printing is by Cadmus Journal Services, Richmond, VA. © 1997 Gemological Institute of America All rights reserved. ISSN 0016-626X SYMPOSIUM Meeting19991999 the Millennium In June 1991, GIA celebrated its 60th Anniversary with the Second International Gemological Symposium in Los Angeles, California. Many of you came to that event and have since told us that these were the most exciting and memorable four days of your career. Having successfully completed the building of our new world headquarters and campus in Carlsbad, California, we are now turning our attention to hosting the Third International Gemological Symposium, June 21–24, 1999, in beautiful San Diego, California. For GIA, this event will mark a seminal moment in the future of the gem and jewelry industry. Focusing on the theme Meeting the Millennium, the Third International Symposium will feature more than 100 speakers and panelists, a “poster session” of reports on cutting-edge research, world-renowned opening and closing keynote speakers, and a special day designed to stimulate the members of our industry to move beyond traditional boundaries to meet the challenges of the new century. We anticipate more than 2,000 par- ticipants in this unique event—a mixture of academic enrichment, gemological insight, cultural experience, and networking opportunities that will be unlike any seen before in our industry. For those who have not yet had an opportunity to visit our new world headquarters in Carlsbad, a full day of tours and other activities is being planned immediately following the Symposium. Although the event is still many months away, I suggest that you mark your calendars now. Reserve your place for the experience of a lifetime. Come celebrate with us and meet the millennium at the Third International Gemological Symposium, June 21–24, 1999. William E. Boyajian President, Gemological Institute of America Editorial GEMS & GEMOLOGY Fall 1997 165 BENITOITE FROM THE NEW IDRIA DISTRICT,SAN BENITO COUNTY,CALIFORNIA By Brendan M. Laurs, William R. Rohtert, and Michael Gray em-quality benitoite has been mined intermittent- Commercial quantities of gem-quality benitoite are known from a single location in the ly from only one region in the world, the New Idria world, the Benitoite Gem mine in the New G district of San Benito County, California. Benitoite Idria district of San Benito County, Californ- was first discovered there in 1907; it was named after the ia. A barium titanium silicate, benitoite is county, as well as the San Benito River, which runs through typically colorless to blue, and is noteworthy the property, and the nearby San Benito Mountain for its high refractive indices, moderate bire- (Louderback, 1909). Stones are strongly dichroic, typically fringence, and strong dispersion. Benitoite violetish blue and colorless. Exceptionally rare gemstones occurs in altered blueschist within serpenti- are colorless and pink. A few orange benitoites have been nite. The gem crystals formed within frac- produced by heat treatment of colorless material. Benitoite tures as a result of the alteration of blueschist has high refractive indices, moderate birefringence, and by hydrothermal fluids derived from the strong dispersion (exceeding that of diamond in some direc- regional metamorphism of the serpentinite. An extension of the historic deposit at the tions), which render the faceted gems comparable in appear- Benitoite Gem mine was discovered in the ance and price to fine sapphire and tanzanite (figure 1). spring of 1997, which should contribute to Although production has been sporadic throughout the life continued stability in price and supply. The of the deposit, hundreds of carats of faceted material are cur- gemological properties of benitoite readily rently available, in sizes up to 2–3 ct. identify it from similar-appearing gems. Benitoite is a barium titanium silicate (BaTiSi3O9) that forms under unusual conditions. At the New Idria district, benitoite is found exclusively in bodies of blueschist within serpentinite. During 1995, the senior author conducted a detailed study of blueschist bodies throughout the New Idria district on behalf of the Kennecott Exploration Company. The main occurrence, the Benitoite Gem mine, was mapped in detail, and other known benitoite occur- rences at the Mina Numero Uno and Victor prospects were ABOUT THE AUTHORS examined. Microscopic benitoite crystals were detected for Mr. Laurs is senior editor of Gems & Gemology, the first time in a blueschist body on Santa Rita Peak. After Gemological Institute of America, Carlsbad, nearly a decade of inactivity, a small prospect containing California ([email protected]). Mr. Rohtert is Manager of Gemstones, Kennecott Exploration minute amounts of gem-quality benitoite was rediscovered Company, Reno, Nevada ([email protected]). along a tributary of Clear Creek; this locality is now known Mr. Gray is owner of Graystone Enterprises, and a as the Junnila claim. More recently, an extension of the partner in Coast to Coast Rare Stones; he lives in Missoula, Montana ([email protected]). Benitoite Gem mine deposit was found in spring 1997, dur- Please see acknowledgments at end of article. ing exploration down slope of the historic pit. The Benitoite Gem mine was referred to as the “Dallas Gems & Gemology, Vol. 33, No. 3, pp. 166–187 © 1997 Gemological Institute of America mine” until the mid-1960s, but it has been variously called the “Benitoite mine,” “Dallas Benitoite mine,” and “Dallas 166 Benitoite GEMS & GEMOLOGY Fall 1997 Figure 1. Because of its beauty, rarity, and singularity of occurrence, benitoite was declared the official California State Gemstone in 1985. This assortment of stones from the Benitoite Gem mine (1.10–4.77 ct) shows the range of colors that are typically encountered in benitoite. All stones were faceted by Elvis (Buzz) and Michael Gray. From the collection of Michael M. Scott; photo © Harold & Erica Van Pelt. Gem mine” (Wise and Gill, 1977). Today the locali- Frazier (1990a) reviewed the controversial history ty is designated as the Benitoite Gem mine by the surrounding the discovery and development of the mine owners, after the notation on topographic mine without making any judgment, and provided a maps of the U.S. Geological Survey which identify comprehensive bibliography (1990b). the site simply as the “Gem mine.” The first documented piece of benitoite to be faceted was brought to a lapidary in the San HISTORY Francisco Bay area by Sanders’ brother. While an Although there is general agreement in the litera- expert from the Los Angeles area proclaimed the ture that benitoite was discovered in early 1907, new material “volcanic glass,” this lapidary called there is considerable controversy about who actual- the stone spinel: “It was too soft for a sapphire, so I ly discovered it. Louderback (1907) originally decided it must be a spinel; [since] that’s the only attributed the discovery to L. B. Hawkins and T. E. other stone there is of that color” (Marcher, 1939). Sanders. However, two years later, he identified The lapidary showed some of the stones to George J. M. Couch (figure 2), a prospector who was grub- Eacret, the manager of Shreve & Co., one of the staked by R. W. Dallas, as having first found a num- largest jewelry stores in San Francisco at that time. ber of mineral deposits that merited further investi- According to Marcher (1939), Eacret checked the gation (Louderback, 1909). Dallas “induced” stone with a dichroscope and found that it was dou- Hawkins to accompany Couch back into the moun- bly refractive, so he delivered a sample to Dr. tains. According to Louderback (1909, p. 333), George Louderback, a mineralogist at the “While out to examine some copper deposits they University of California (Berkeley) who determined happened upon the benitoite deposit and each that it was a new, undescribed mineral. claims to be responsible for the discovery.” In 1961, In the company of Mr. Eacret, Dr. Louderback Couch’s eldest son, Oscar, published The Benitoite made his first visit to the mine on July 19, 1907, Story, in which he outlined the reasons why his and confirmed the discovery. He revisited the father should be given sole credit finding the deposit on October 11 of that year to study the geol- deposit.

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