This attractive rock is called anyolite, or corundum-amphibole zoisitite (or corundum-amphibole zoisite metamorphite). Anyolite is a metamorphic rock consisting of finely-crystalline chromian zoisite (green, Ca2Al3(Si2O7)(SiO4)O(OH) - calcium aluminum hydroxy-oxysilicate with Cr impurity) with minor chromian amphibole (black, apparently either chromian tschermakite and/or chromian edenite) and some large porphyroblasts of red corundum (ruby) (Al2O3 - aluminum oxide with Cr impurity). There's also minor Ca-rich plagioclase feldspar (anorthite, CaAl2Si2O8) in this rock.
Locality: Mundarara Mine, ~27 km west of Longido, northeastern Tanzania, southeastern Africa.
Origin: Published mineralogy studies indicate that this chromian zoisite-ruby combination is the result of very high-grade metamorphism of anorthosite, an intrusive igneous rock dominated by Ca-rich plagioclase feldspar. The chromium (Cr) in the zoisite and the corundum (ruby = corundum with chromium impurity) is derived from metamorphic alteration of chromite crystals (FeCr2O4 - iron chromium oxide) in the original anorthosite unit. Chromite and chromitite (= chromite-dominated igneous rock) are commonly associated with anorthosites in LLIs (= large layered igneous intrusions, such as Montana's Stillwater Complex).
Geologic Context & Age: This Tanzanian anyolite is hosted in gneisses exposed in the Mozambique Collision Belt, an ancient, north-south trending, tectonic collision zone in eastern Africa. It dates to the Pan-African Orogeny (Neoproterozoic), during which the ancient continents of West Gondwana (~modern-day South America & Africa) and East Gondwana (~modern-day India-Australia-Antarctica) collided, forming the long-lived, small supercontinent Gondwana.
Anyolite (= corundum-amphibole zoisitite) (4.7 cm across at its widest) from the Neoproterozoic of the Mundarara Mine, northeastern Tanzania. Green = Cr-zoisite; black = amphibole; reddish = ruby (corundum).
Mostly synthesized from Game (1954), Mercier et al. (1999), and info. provided by Alan Wright.
Game, P.M. 1954. Zoisite-amphibolite with corundum from Tanganyika. Mineralogical Magazine 30: 458-466.
Mercier, A., P. Debat & J.M. Paul. 1999. Exotic origin of the ruby deposits of the Mangari area in SE Kenya. Ore Geology Reviews 14: 83-104.