In the summer of 1998, natives along the Ioshy-Tulear Road, close to the cliffs of the Isalo National Park, discovered sapphire
in the small Ilakaka River. Although most of the finds were of low-quality, sapphire
and other gemstones were found abundantly at many spots, not only in the Ilakaka River, but over the whole region. The productive area was found to stretch from north to south for 200 km, encompassing the area between the Isalo mountains in the east and Sakaraha in the west.
A diverse range of gemstones occur in sedimentary rocks: corundum
(colourless, sky-blue to deep-blue, green, purple, bright-yellow and pink, rarely red); colourless to sky-blue topaz
in every colour; mainly red, brown and violet zircon
; pink, violet, green, bluish and red spinel
; reddish to purple garnet
(bright, yellow-green, cat's eye and alexandrite
; colourless, light-blue and light-pink beryl
; light-blue to dark-blue kyanite
; reddish-brown andalusite
; colourless, pink quartz
Gigantic quantities of rock were reworked by the erosive processes that covered extensive parts of eastern Madagascar; thus over millions of years, elongated gemstone placers formed the riverbeds having been derived from the most diverse sources of metamorphic, pegmatitic and pneumatolytic origin. Minerals of all colours and a great variety of parageneses were thrown together and mixed in the sediments. Ilakaka is a good example of a true paleo-placer, among the largest known on Earth.
Ref: Madagascar - extraLapis English No.1, 2001, pp. 90 - 93