Tanzania is well-known for being a treasure-trove of gems thanks to the Mozambique orogenic belt (a geological formation that acts as nature’s treasure box). The mines in the country have been worked on and off for many years, but after very fine, large, and untreated rubies appeared from Winza (roughly located in the center of the country) in 2008. Multiple rubies were over ten carats and a gem rush began.
Note that chromium, the color-inducing element in rubies, inhibits crystal growth. This makes large rubies extremely rare to find, and is why the 10-carat rubies are very notable and expensive.
Most of the gem material recovered from the mines in the country are a variety of sapphires, and very rarely a padparadscha sapphire of lower quality. Rubies make up a smaller, though notable portion of the yield.
Rubies from this area commonly have undesirable blue color-zoning, though top-quality, perfectly red and very clear rubies are possible from the source. They are also basalt-hosted, meaning they often have features like higher clarity and being a little darker (partly due to lack of fluorescence).
R3698 | right | medium | “Ruby ID: R3698”
Common appearance of a Winza Ruby.
Photo by Ted Themelis
Tanzania’s established ruby mines are located primarily in the northern (Longido and Lossongonoi) and central (Morogoro) parts of the country. These mines have been worked on and off for decades.
Around 2003, exciting reports of fine quality ruby also emerged from the Province of Rukwa in the southern highlands, an area known mainly for its remoteness. Small-scale operators are responsible for a majority of the mining activity in Tanzania, although a large-scale operation is working the Lossongonoi deposits.
Most ruby deposits have been worked to depletion very quickly, though there are a few still in production. There are many other gemstone deposits that are being worked here like sapphires, tourmaline, spinel, tanzanite, and different species of fine-quality garnets.