Feature Gems & Gemology, Winter 2002, Volume 38, No. 4

Rhodizite-Londonite from the Antsongombato Pegmatite, Central Madagascar

Brendan M. Laurs, Federico Pezzotta, William B. (Skip) Simmons, Alexander U. Falster, and Sam Muhlmeister

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Londonite was approved as a new mineral species in 1999. This rare aluminum-beryllium borate is the cesium-rich analogue of rhodizite; the two end members can be separated only through quantitative chemical analysis. The only known source of gem-quality londonite is the Antsongombato pegmatite in central Madagascar, from which at least 300 carats of rhodizite-londonite have been faceted. The gemstones are colorless to greenish yellow, with an R.I. range of 1.689–1.691, S.G. of 3.34–3.42, and yellowish green fluorescence to short-wave UV. Internal features include partially healed fractures and conspicuous growth zoning that typically correlates to areas of anomalous birefringence. Some near-colorless rhodizite-londonite from Antsongombato has been irradiated to greenish yellow or yellow using X-rays. Both natural-color and irradiated rhodizite-londonite will fade on prolonged exposure to sunlight.