Best-known for it's emeralds, the deposit is in Mayoka Village at the southern tip of Lake Manyara, regularly producing high quality alexandrite, but most of the stones appear to be khaki or yellowish-green in daylight, moving to brownish-red under incandescent light.
The far-flung emerald deposit near Mayoka Village, the deposit is at the southern tip of Lake Manyara reported small pieces of mother rock in contact zones between biotite-actinolite schists/gneisses and pegmatites, on which, associated with feldspar and mica, alexandrite, emerald and ruby occur. Production statistics indicate that over one thousand carats of alexandrite have been mined, but no official alexandrite reserve exists. It is believed that the western sides of Lake Manyara host some alexandrite deposits. Recent discoveries of alexandrite and cat's eye are being worked by small-scale and artisanal miners in the beds of the Muhuwesi, Mtetesi, Lumesule and Nambalapi Rivers.