Pegmatites of the Chrysoberyl Subtype, outcropping east of the southeastern shore of the lake. These pegmatites contain miarolitic cavities lined with crystals of smoky quartz
and superficially transparent, K-feldspar crystals that are either corroded or perfectly preserved. Well-formed, locally abundant, black tourmaline
is present in either the massive core quartz or in the pockets. Chrysoberyl
occurs most often in the miarolitic cavities as completely formed twinned-gemmy crystals floating in the clay or more rarely, as twinned crystals growing on quartz prisms or on faces of tourmaline crystals. The largest crystals of chrysoberyl
from miarolitic cavities are up to 5 cm, though some locals have reported finding crystals, in the past, of up to 10 cm in diameter.
Though rare, large good specimens of milky to gemmy, blue-green beryl
have been found in the cavities in the past.
Ref: Madagascar, extraLapis English No.1, 2001, p. 47