History · Geology
About Merelani Hills
The Merelani Hills near Arusha in northern Tanzania are the unique world locality for tanzanite — the violet-blue gem variety of zoisite — and one of the great metamorphic gem provinces. Discovered in 1967 and now a controlled mining zone, Merelani is the source of essentially all gem tanzanite, plus large emerald-green tsavorite garnet, and gem prehnite.
Geology
The deposit is hosted in graphite-rich gneisses of the Mozambique Belt — a Pan-African (~600 Ma) high-grade metamorphic terrane. Gem pockets form within boudinaged graphitic schists where late retrograde fluids deposited V-bearing zoisite (tanzanite) and Cr-V-bearing grossular (tsavorite).
Notable Minerals
Tanzanite (the world’s only commercial gem source — typically heat-treated to enhance violet-blue), tsavorite garnet (Cr-V-bearing grossular, world-best gem green grossular), prehnite (gem-grade gel-textured green crystals), diopside (chrome diopside), graphite (large flexible flakes), calcite. Some pockets yield colorless to gem-pink zoisite.
Collector Notes
Tanzanite supply is government-restricted; rough is increasingly scarce. Tanzanian government licenses block-by-block mining. Top-tier 5+ ct violet-blue tanzanite gems command four-figure-per-carat prices.
