ABSTRACT
The deposits in India are all secondary or tertiary and the diamonds were obtained from ancient alluvial conglomerates and related deposits as well as from recent river gravels. By the early 1700’s the diamond fields of India were in decline but in 1725 diamond was discovered in the Diamantina region of eastern Minas Gerais State in Portuguese Brazil. The methods of mining were somewhat similar to those of India in as much as the diamond-bearing gravels or soft rock were washed and the resulting material sieved and sorted for diamond. In places the diamonds could be simply picked from the desert surface as a result of the wind having blown away the less dense and finer material – a geological process in the formation of what is referred to as lag gravel, or desert pavement. The discovery of microscopic diamond in garnet in metamorphic rocks in Khazakstan has caused rethinking of crustal processes among some geologists.
