Alluvial diamondsUntil the carrot shaped kimberlite pipes and their surrounding erosion remnants wer

Alluvial diamondsUntil the carrot shaped kimberlite pipes and their surrounding erosion remnants were discovered in South Africa late in the 19th century, all the known sources of diamonds were alluvial deposits, precipitated by the balance between the energy erosion (usually rivers), mineral density and palaeotopography. Before the New World was conquered the only known sources were in India (the famed mines of Golconda) and on Borneo, in both cases the eroded remnants of old kimberlites deposited as pebbles in river sediments. When the Portuguese fanned out across Brazil in search of mineral treasures, gold miners found strange pebbles while panning near the present town of Diamantina. While no one recognised what they were for some time, they did come to the world’s attention, and provided the first regular and reliable source since medieval days, prompting the early development of the diamond cutting industry in Europe.They occur in sedimentary rocks called conglomerates (once known by the more poetic name of pudding stone, see http://on.fb.me/1Kd83hN), in which pebbles/cobbles of varied kinds and angularities have been jumbled together and cemented, usually in a fine grained matrix (eg sand) or precipitated mineral (eg quartz or calcite). In the 10 cm specimen from the Formosa River in Bahia, angular clasts of a pale mineral sit next to a rounded dark pebble, with a half centimetre white diamond poking out between them. The original kimberlite pipes have not been found, having been long removed from the geological record.LozImage credit: LGF Foundation -- source link

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