I have spent over 7 years working as a geologist in East Africa, focusing on the exploration and mining of both the blue-violet variety of Zoisite, named Tanzanite, and the green Grossular Garnet, named Tsavorite, respectively. Here I present some of the origins of both present day and past, major and minor sources of the green Grossular Garnet. The locations shown are both exploration areas and mines that I have worked at, and personally seen both mineral and/or gem green Grossular Garnet.
Here is a summary of the origins 1-7 (number; country/area/owner; description; mining* style):
1 – Tanzania; Merelani (from the Tanzanite Mines); light green tone (mint) garnet, both mineral and gem material; Underground
2 – Tanzania; 16km north east of Merelani; light to medium tone green garnet, only mineral seen; Underground
3 – Tanzania; Shamberai Area; Exploration area, historic underground mining (only origin I never saw any material)
4- Tanzania; Lemshuku Area; light to medium tone, medium saturation green garnet; mineral and gem material; Open Pit
5 – Tanzania; Lemshuku Area (Saul’s Tsavorite Mine); light, medium and dark tone, generally medium saturated green garnet; mineral and gem material; Open Pit
6 – Tanzania; 14km south of Lemshuku Area; medium to dark tone, medium to high saturation green garnet; mineral and gem material; Underground
7 – Kenya; Tsavo (Bridges Tsavorite) – light to medium dark tone, medium to high saturation green garnet; mineral and gem material; Underground
Geologically, what is consistent across the border of the two countries is that all the green garnet is forming in the graphite-rich schists and gneisses of the Neoproterozoic Pan-Africa Orogenic Belt. I have seen green grossular garnet form in two completely different rocks types both containing graphite from the Belt. The importance of the graphite is significant, as this is the source of the Vanadium, the trace element giving the Grossular Garnet its green color. And like in all gems, the amount of Vanadium, does vary resulting different tones and saturations.
Is all of the gem green Grossular Garnet, Tsavorite? I am both a geologist and gemologist, but I am guided by what I was taught at the GIA, and that when a gem has a tone of 4 and above (out of 7), then it is Tsavorite Grossular Garnet, irrespective of geographical origin.
*mining includes small scale, unsupported, blasted shafts up to 20m underground, as well large scale, supported, shafts beyond 20m underground.