The Sinkankas Symposium – Tanzanite & Tsavorite

The eastern margin of Africa, southern Madagascar, northwestern Pakistan, southern India, Sri Lanka, and eastern Antarctica are considered relics of the Neoproterozoic Metamorphic Mozambique belt (NMMB), the so-called Mozambique gem belt. Along this north-south trending orogenic belt, many varieties of gemstones occurrences are found, including ruby, sapphire, emerald, diamond, tourmaline, spinel, and alexandrite, as well as garnet varieties such as demantoid, color-change garnet, hessonite, and spessartine. However, only two gemstones are unique to this belt: tanzanite and tsavorite. Tanzanite, the blue-violet variety of zoisite, and tsavorite, a green variety of garnet, share a common location, time of discovery – 50 years ago – and most importantly genesis of formation. This talk will present an overview of the chemistry, mineralogy, and geology of formation of tanzanite and tsavorite.

This presentation will tell the story of a 10-year quest to build a private collection of the finest tanzanites and unheated zoisites. There will be documentation of sources, details about rough selection, and technical discussions of the cutting process. This will include rough orientation and cutting techniques.

The son and wife of the legendary geologist Campbell Bridges will offer a firsthand account of the history of tsavorite and his adventure-filled life. Renowned for his discovery of tsavorite, Campbell Bridges was also Tiffany & Co.'s consultant geologist on tanzanite and the person who first brought it to the United States for identification. His geological expertise allowed him to lead successful, if often perilous, quests for some of the world's rarest gemstones in the uninhabited bush of East Africa. His buccaneering life featured frequent brushes with snakes and scorpions, big cats and buffalo, and of course, his discovery of tsavorite in the Tanzanian bush, in 1967.

Bruce will also discuss the evolution and development of the famed Scorpion mine and the tsavorite mining industry over the past five decades, up to and including the present-day mining landscape in Kenya. In addition, he will show the company's fully integrated operation, which includes every aspect from mining, sorting, grading and cutting to marketing and sales. For the first time in her storied career Judith Bridges will present and relate never-before-heard experiences working by Campbell Bridges' side.

Tanzanite is the blue-violet gemstone variety of the mineral species zoisite, found only in the northeastern part of Tanzania, near the town of Merelani. Discovered in 1967, very little geology work and research was conducted on the deposit until 1998, when Block C began to be mined in a formal and professional manner. This is significant, because Block C consists of the largest and the central 2 km² section of the known 7 km² deposit, and also because for the first time there was significant surface and underground rock exposure, outcrop availability, and drill core.

The geological history of the deposit and the formation of tanzanite is complex. This region of the Mozambique Belt dates back to approximately 1000 Ma (million years ago) and has been subjected to multiple phases of high-grade metamorphism and granulite facies conditions (10–12 kbar and 850–1000°C). Crustal uplift resulted in multiphase retograde deformation events and the crucial tanzanite-forming event of the Pan African at ca. 550–500 Ma in amphibolite/greenschist facies conditions (6–7 kbar and 520–600°C).

The complex deformational history and geochemical events played critical roles in the mineralization processes. Calc-silicate skarn layers were created in the stratigraphic sequence through the metamorphism and metasomatism of calcium-rich boudinaged zones and depleted graphitic gneiss horizons. V-rich green grossular garnet crystallized in tension zones during prograde metamorphism.

Tanzanite mineralization occurred during the retrograde period ca. 585 ± 28 Ma at P-T conditions estimated at 5–6 kbar and 650 ± 50°C through two different processes. One was the recrystallization of grossular garnet during the retrograde phase, and the other was migration and crystallization of V and Ca-enriched fluids.

This presentation will offer the collector's perspective on evaluating and obtaining the finest natural crystals of these two gemstones.

This presentation will discuss the known treatments of tanzanite and tsavorite. There are very few treatments done to tsavorite, but tanzanite has several possibilities. In addition to treatments, we will also discuss the various imitations that have shown up over the years and how you can recognize them.

In this presentation, a variety of tanzanites and tsavorites and their different inclusion scenes are examined, revealing the unique stories they have to offer.

Few gems have achieved the prominence of tanzanite in the international market. During the 1990s and into the early 2000s, tanzanite was one of the most popular gems in the Western market. For a period of time, dealers recognized this status, often referring to ruby, sapphire, emerald, and tanzanite as the Big Four. Unlike the prior three, tanzanite is generally accepted to be limited to a single location—Merelani, Tanzania. As such, tanzanite is far rarer than most gemologists realize. Over the course of five decades, few gems have experienced the price and market volatility of tanzanite. Yet during that timeframe, this gem has traded within a narrow price range, having established a well-defined price floor and ceiling.

This presentation will examine key price influencers for tanzanite observed during various points along the gem's 50-year history. The presentation will conclude with a few comments regarding future demand and price projections for this best-known zoisite variety.

Tsavorite's color comes from a combination of vanadium and chromium. Vanadium content can commonly range from a few tenths of a percent to about three percent, while chromium concentrations are commonly an order of magnitude lower. However, some garnets from the tsavorite mines have up to five to seven times more chromium than vanadium. Unlike the case for ruby and spinel, which are red due to their chromium content, both vanadium and chromium cause green color in tsavorite garnets.

The variety of zoisite known as tanzanite, when in its natural state, can be a spectacularly pleochroic mineral displaying different colors when viewed down the a-, b-, or c-axis or when viewed in linearly polarized light. Examples of these spectacular colors will be illustrated. Many tanzanite specimens also show a significant color change when heated at several hundred degrees. These colors principally derive from tanzanite's vanadium content. Other colors of zoisite also occur in and around the tanzanite mines, ranging from yellow and golden orange to pink. For example, pink color is associated with low vanadium and elevated manganese concentrations. Spectroscopic and chemical studies help to determine the metal ions responsible for the color and their oxidation states.

Dr. Raquel Alonso-Perez is curator of the Mineralogical and Geological Museum, Harvard University (MGMH). Raquel received her B.S. in geology from the University of Granada, Spain, and her Ph.D. in earth and material sciences from the Swiss Federal Institute of Technology (ETH) in Zurich. She also obtained an FGA diploma from the Gemological Association of Great Britain (Gem-A). Her teaching strengths are mineralogy and gemology, and her main research interests are mineralogy and the genesis of gem deposits by using a combination of non-destructive analytical techniques. As curator of the MGMH, Raquel is responsible for access to and continued development of the earth science collections as a research and public education resource. Raquel serves on the advisory board of the Association for the Study of Jewelry and Related Arts, the Boston chapter of the Women's Jewelry Association, and associate editor of the Journal of Gemmology.

Meg Berry has been cutting gems since 1975. She studied faceting at Grieger's Faceting School, and earned a Master Faceter degree in 1977. Meg has faceted professionally for major clients in the gem industry for more than 40 years, although her interest since 2001 has expanded to include carving gemstones. Meg has won nine AGTA Spectrum Awards, seven for faceting and two for carving, most recently first place in the 2017 event.

Bruce and Judith Bridges are the son and wife of Campbell Bridges, the discoverer of tsavorite and the person who brought the first tanzanite to the United States for identification. Campbell Bridges was also Tiffany & Co.'s official consultant geologist on tanzanite. Bruce has been intimately involved in the gem industry since childhood, working side by side with his father. He has completed extensive fieldwork in the Taita/Taveta region of southern Kenya. As president of Bridges Tsavorite, Bruce carries on the family's three generations of tradition as gemstone miners and dealers in Africa. Judith Bridges also worked closely with her husband for more than 40 years and has been involved in every aspect of their gem ventures throughout her career. The family business is headquartered in Nairobi, with two wholesale distribution offices in the United States. The company is proud to maintain its fully integrated operation, which includes every aspect from mining, sorting, grading and cutting to marketing and sales.

Robert Gessner is both a colored gemstone geologist and a gemologist by profession. He completed his bachelor's degree in geology at the University of Cape Town (UCT) in 2002 and completed his post-graduate Honors Degree at the University of Stellenbosch in 2004. His first professional exposure to the colored gemstone industry was in Tanzania in 2003, and this was to become his specialization and career. Robert has more than 15 years of colored gemstone geology, mining, and consultancy experience in East Africa, working specifically with tanzanite, tsavorite and Zambian emerald. During that time, his involvement with the deposits' geology and mining and his operational input played an integral role in the world's two largest colored gemstone mining companies' successes between 2006 and 2015. In 2016, Robert received his Graduate Gemologist (GG) diploma from GIA in Carlsbad. Today he is based in Los Angeles and has his own business (www.gessnergems.com) providing consulting, training, colored gemstone sourcing, and sales through his curatorship of fine gemstones.

Bill Larson began his interest in gems and minerals at a very early age, when he combed the hills of San Diego County with his father and Josephine Scripps and John Sinkankas in search of fine crystals. He earned an advanced degree in geological engineering from the Colorado School of Mines.

In 1968, Larson and then-partner Ed Swoboda purchased three gem mines in the Pala mining district—the Tourmaline Queen, Stewart Lithia and Pala Chief, whose histories date to the 1800s. Larson stepped onto the international stage in 1972, when an extraordinary strike established his company and the Pala mining district as one of the world's premier sources of tourmaline.

As president of Pala International Inc., Larson heads one of the industry's leaders among gemstone import, lapidary and mining operations internationally. In a joint venture with his wife, Bill and Jeanne Larson own The Collector, a retail shop located in Fallbrook, which showcases fine colored stones, jewelry and objets d'art. Today, Larson travels the world as a guest lecturer, media spokesperson, and authority on minerals and gems, the mining industry and colored stone pricing.

Will Larson is a mineral dealer and collection adviser from San Diego, California. He has been in the industry since he was a young boy participating in the family business. During that time, he was exposed to mining, brokering and marketing of minerals and gemstones. At the age of six he started his own section in Pala International's booth called "Will's Corner," where he would sell minerals he found himself. Will began working fulltime for his father at Pala International in 2009 but started doing trade shows in Tokyo by himself as early as 2005. He has never missed a Tucson Gem and Mineral Show (according to his parents). His first major overseas trip to buy minerals and gemstones was in 1997, at the age of 12, to Vietnam, Thailand and Burma. His first trip to Tanzania was in 2012 for the first Arusha International Gem Jewelry and Mineral Fair. At that time he visited both Tanzanite One's operation and the artisanal miners in the D block. At Tanzanite One, he went deep into the mine to search for tanzanite and to better understand the location and rarity of these wonderful crystals. He has always been fascinated by natural tanzanite. After witnessing the incredible effort required to obtain the gem, he redoubled his efforts and since then has supplied collectors and dealers with hundreds of tanzanite crystals.

Shane F. McClure is global director of identification services for GIA, based in Carlsbad, California. A frequent speaker at national and international conferences and trade shows with more than 38 years' experience in the field of gemstone identification and treatments, he has been a contributing author on many articles published in Gems & Gemology and other publications. He has won Gems & Gemology's Most Valuable Article Award eight times for reporting on topics such as diffusion-treated sapphires, the identification of filled diamonds, and gemstone enhancement and detection in the 1990s. He has twice coauthored articles that have won the American Gem Society's Richard T. Liddicoat award for outstanding journalism in the jewelry trade. In 2007, he was the recipient of the Antonio C. Bonanno Award for Excellence in Gemology. Mr. McClure is also a co-editor of the Lab Notes section of Gems & Gemology and an accomplished gem and jewelry photographer and photomicrographer.

Nathan Renfro, a native of western North Carolina, developed an interest in minerals during his late teens, which was sparked by his grandfather's rock collection. In 2006 he completed his undergraduate studies in geology and then went on to enroll at GIA for the resident Graduate Gemologist (GG) program as a recipient of the William Goldberg Diamond Corporation scholarship. Mr. Renfro also completed his FGA in 2014.

After completion of the GG program, he was hired by the GIA laboratory as a diamond grader and soon transferred to the Gem Identification department in 2008. Since then, Mr. Renfro has authored or co-authored more than 100 gemological articles and lectured to gem and mineral groups throughout the Unites States. His primary areas of gemological interest are inclusion identification, photomicrography, gemstone cutting and defect chemistry of corundum. Mr. Renfro is currently the Manager of the gem identification department and also is a microscopist in the Inclusion Research Department. Mr. Renfro is also the editor of the G&G Microworld quarterly column, which is published in GIA's award-winning professional journal Gems & Gemology, for which he is also a member of the editorial review board.

Dr. George R. Rossman is Professor of Mineralogy in the Division of Geological and Planetary Sciences at the California Institute of Technology in Pasadena. His principal research interests deal with the use of spectroscopic probes to study minerals. His work addresses the origin of color in minerals, methods for micro-phase identification, the long-term effects from the exposure of minerals to background levels of natural radiation, and X-ray amorphous minerals, including biominerals and weathering products. An important application of his studies concerns the role of low concentrations of water and hydroxide in nominally anhydrous solids. He and his students develop analytical methods for OH analysis and examine the mode of incorporation of hydrous components in solids and their role in modifying physical and chemical properties. Prof. Rossman was the recipient of the Mineralogical Society of America's inaugural Dana Medal in 2001, the Richard P. Feynman Prize for Excellence in Teaching at Caltech in 2004, and the Friedrich-Becke Medal of the Austrian Mineralogical Society in 2005. Rossmanite, a species of the tourmaline family, is named after him. Prof. Rossman joined the Caltech faculty in 1971. He received a B.S. in chemistry and mathematics from Wisconsin State University, Eau Claire, where he graduated summa cum laude. He holds a Ph.D. in chemistry from Caltech. Prof. Rossman has authored or coauthored more than 320 publications in the mineralogical and chemical sciences.