RAPAPORT... Gübelin Gem Lab Ltd., Lucerne, Switzerland
In November 2007, news started to spread in the Tanzanian gem-mining community that a promising new ruby deposit had been found near the village of Winza, in the Mpwapwa district of the Dodoma region. The rumor triggered a ruby rush and, within a few days, miners from all over Tanzania were on the road to the new deposit. In January 2008, the Gübelin Gem Lab received a report from Abdul Msellem, a Tanzanian gem broker who had just returned from a one-month mining expedition to Winza. He reported that approximately 600 miners were already working the new deposit, which was producing not only pink to red rubies but also blue and purple sapphires. According to Msellem, the site of the find, a former cornfield, was yielding small stones of facetable quality from 0.5 to 3 grams and well-formed crystals up to 10 grams.
Subsequently, the Gübelin Gem Lab received some fine-quality rubies said to have come from the Winza area. The lab found that the best-quality stones were characterized by exceptional clarity and a very fine red to pinkish red color reminiscent of Thai rubies. However, the most striking feature of the best of these rubies was their highly transparent crystalline clarity.
The newly discovered Winza rubies were first presented to the world market in April at BaselWorld 2008 by gem dealers from Thailand and Sri Lanka. According to the dealers, recent production levels of the finest-quality rubies at the site appear to have fallen, compared with the output of the first three months, despite the presence of more miners. However, as is usual in such remote locations, reliable, quantitative data is not readily available.
It is also reported that the top-colored rubies — such as those seen at the Gübelin Gem Lab during BaselWorld — were found exclusively in a residual deposit in the soil at the mining site. So far, such fine red rubies have not been found in the solid rock underneath the gem-rich soil and miners continue their search for the host rock of the top-quality rubies.
EXCEPTIONAL CLARITY
When the Gübelin Gem Lab got its first look at the Winza rubies, the relative absence of typical inclusions in the stones was so impressive that it raised doubts as to their origin and led the lab’s experts to suspect that the samples were possibly a new type of synthetic ruby. In order to authenticate the stones’ natural origin and to obtain samples firsthand at the mine, the lab dispatched a field expedition to Winza. In addition to collecting samples for the Gübelin reference collection, the field expedition also was to gather information about these new rubies, which present some gemological characteristics unusual for rubies, including curved tubes and healing fissures (see “Gemological Description,” below).
The field expedition took place from April 15 to 23, 2008, just after BaselWorld. During the expedition, three days were spent at the mining site and visiting the local gem trading centers in the towns of Mpwapwa and Arusha. It was learned that the area has long been known for mining activity. In fact, a map dating from the 1950s and hanging in the office of the Mpwapwa police chief, shows a mine symbol at the exact location of the current ruby mining area.
In Tanzania, foreign buyers are not allowed to work at, or even visit, mining areas, so most of the foreign buyers have to wait in Mpwapwa for the stones collected at the mines by Tanzanian brokers. The town of Mpwapwa is located about a two- to five-hour drive from the mines, depending on the weather and the condition of the dirt road linking the town and the mines. Most of the buyers in Mpwapwa are from Chanthaburi in Thailand. Some Sri Lankans, Indians and even a French buyer are also competing to buy the new Winza rubies.
Buying gems in Mpwapwa is also reportedly complicated by the fact that many synthetic stones are being mixed in with the natural stones. Without proper instruments and at remote locations, the separation of natural stones from synthetic stones is not always easy.
THE SITEThe Winza mining camp is situated on the eastern side of the Winza River on the slope of the Mtakanini hill, about six miles south of the Winza village. Gem mining is carried out around the wood and plastic-sheeting huts of the temporary village, in which about 4,000 people have settled since December 2007. Most of the mining operation visited by the Gübelin expedition was located on the hillside less than 1,300 feet from the mining camp.
Mining operations at Winza appear to have been well organized by experienced, professional miners, reflecting Tanzania’s 100 years of experience in gem mining. Many people from the area applied for mining licenses and began working immediately. Officials from the Dodoma Regional Mining Department advise that these applications for mining licenses are currently being examined and processing could take a few additional months.
It appears that most of the surface soil has been worked at the mine already and, consequently, more miners have begun to mine the underlying hard rock dikes, which apparently bear only corundum of lower quality. In just a few weeks time, tunnels of up to 32 feet in depth were dug, by hand, into the hard rock without benefit of power equipment. Black, probably lamprophyric, dikes contain purplish, pinkish and blue sapphires in association with garnets. The main mineral phase in this rock was later determined by the Gübelin Gem Lab to be a hornblende amphibole.
THE MATERIAL
The first results of the research conducted on the material collected by the field expedition reveal that the finest- quality Winza rubies show some similarities to the lower-quality rubies and pink, purple or blue sapphires from the same area, but also exhibit some differences (see “Gemological Description”). Earlier published reports on the Winza discovery considered only the top-quality rubies, while the more abundant, lower-quality stones have not so far been described. However, these lower-quality stones constitute the main production coming out of Winza today, as the gem-rich soil has been almost completely worked out. Compared to only a few weeks ago, buyers today are experiencing greater difficulties in finding fine-quality Winza rubies and, during the Gübelin expedition, no signs were observed of a systematic exploration being undertaken. Still, the fact that the surfaces of the top-quality rubies found in the soil are only slightly abraded might be considered an indication of a primary deposit located nearby.
A sustainable supply of top-quality Winza rubies for the global gemstone market will depend on the existence, and discovery, of such a deposit. Together with other researchers, the Gübelin Gem Lab is undertaking an extensive and comprehensive gemological and geological study of the new material, including detailed spectroscopic, chemical and isotopic analysis. The results of this research will be published at a later date.
GEMOLOGICAL DESCRIPTION: RUBIES FROM WINZA, TANZANIA
Winza rubies and sapphires range in color from red to purple, purplish red, pink and blue. High-quality stones in large sizes up to 15 carats cut have been found, with an intense red color that recalls the color of the best Thai rubies and with a very high transparency.
The internal features of Winza rubies are quite different from rubies of other well-known, marble-related deposits in Burma, Afghanistan, Vietnam, Tajikistan and the basalt-related deposits in Thailand.
The chemical analysis of Winza rubies, using LA-ICPMS and ED-XRF, revealed relatively homogeneous properties for all the Winza material, with a combination of high levels of iron and very low levels of titanium, vanadium and gallium.
Inclusion features are distinct for these rubies. While rutile needles have not been found as yet in any Winza material, the inclusion image of the top-quality, deep-red material is quite different from that of the more pinkish, purplish or blue lower-quality stones. In the latter, twinning planes, straight intersection tubes, folded healing fissures, crystal inclusions of amphibole and garnet, and negative crystals are more common.
In fine red Winza rubies, such inclusions are very rare. However, the top-quality rubies show some other locality-specific inclusions.
Long tube-, fiber-, needle- or hair-like inclusions are found. They are slightly curved or bent, and are filled with a brownish — probably polycrystalline — solid material. Such inclusions have so far not been reported in other natural or synthetic rubies. Growth structures are also common in the form of straight bands, sometimes showing a slightly blurred appearance, in association with blue-color bands. Blue-color zoning and banding is very abundant and distinct in Winza corundum. Healed fissures with coarse texture and irregularly shaped constituents are also found, similar to flux in some synthetic rubies. The correct interpretation of these healing fissures can be challenging for gemologists. In addition, the low levels of titanium, vanadium and gallium that are typical for Winza rubies are also known in some synthetic flux rubies. Advanced analytical testing is therefore required to reliably separate clean, natural Winza stones from synthetics.
While solid crystal inclusions are rare in fine Winza rubies, the Gübelin Lab has, by means of Raman spectroscopy, identified amphibole and garnet. This is not unexpected, as the gems were found in situ in an amphibole rock, in association with orange garnets.
Article from the Rapaport Magazine - July 2008. To subscribe click here.