Ruby Zoisite: Properties, Facts and Photos
What is Ruby Zoisite?
Ruby zoisite is a metamorphic rock composed of red corundum crystals embedded in green zoisite. Inclusions of black amphibole, commonly called black hornblende, are often present. Analysed samples of the black amphibole from Longido, northern Tanzania, have been identified as pargasite.
Due to its striking appearance, ruby zoisite is widely used for decorative purposes. However, the difference in hardness between the corundum and zoisite can make cutting and polishing challenging.
On the Mohs scale of hardness, zoisite grades 6.5 to 7, while corundum grades 9. Diamond, the hardest mineral on Earth, grades 10.
Red corundum is called ruby, while gem-quality corundum of all other colours is known as sapphire. The ruby in ruby zoisite is mostly opaque and unsuitable for faceting.
Traces of chromium give the ruby and zoisite their green colour. The same element produces these two contrasting colours because it affects how light is absorbed differently within each mineral's crystal structure.
Most ruby zoisite comes from the Longido district of northern Tanzania. A major source is the Mundarara mine, and several small-scale mines nearby are operated by the Maasai.
The name 'anyolite', which is also sometimes used, comes from the Maasai word 'anyoli' meaning 'green'.
Ruby zoisite from Longido is associated with ancient metamorphic events around 500 to 650 million years ago.
It has also been found in Kenya.
Although often described as a mineral, ruby zoisite is a rock because it's composed of several minerals, including corundum, zoisite and amphibole.
Ruby zoisite is often mistaken for ruby in fuchsite, which can look very similar. Ruby in fuchsite is mined in southern India.
Fuchsite, a type of green mica, is much softer than zoisite.
Longido Ruby - The Red Corundum in Ruby Zoisite
In 1949, prospector Tom Blevins discovered a ruby-bearing outcrop near Longido, northern Tanzania.
According to an account by Edward Swoboda, Blevins found loose, well-formed ruby crystals in a shallow basin below an outcrop of green rock. Other crystals remained partially embedded in the weathered zoisite, their red colour standing out against the green host rock.
Blevins believed the find would make him and his partner wealthy. However, ruby suitable for faceting occurred only at one end of the outcrop, and most of the crystals were opaque. They spent years trying to market the rubies without much success.
Significant amounts of Longido ruby were initially found near the surface. However, mining later extended underground, with deep tunnels used to reach the ruby-bearing rock.
The name 'ruby in zoisite' is widely used around the world and describes how the ruby occurs within the rock.
Longido is located in the Arusha region of northern Tanzania, near the border with Kenya. Arusha is often used as a base for those who climb Mount Kilimanjaro. It's also part of a broader gemstone-rich area, which includes the Merelani Hills, where tanzanite is found.
Article Pictures
The ruby zoisite in the picture at the top of our article is in the Smithsonian National Museum of Natural History, Washington, D.C. Photo: Stone Mania.
The third and fourth pictures are courtesy of Stan Celestian.
Pop-up photos
Pargasite, ruby zoisite: Courtesy of Géry Parent. The ruby in fuchsite stones are from our collection.
The photos in our article and the links for the pop-up photos link to the original image on Flickr. You may only be able to view them if you join. Joining Flickr is free, with no additional charges.
