The diamond is found under three conditions: (1) in the gravels of the present rivers, embedded in a ferruginous claycemented conglomerate known as cascalho; (2) in terraces (gupiarras) in a similar conglomerate occupying higher levels in the present valleys; (3) in plateau deposits in a coarse surface conglomerate known as gurgulho, the diamond and other heavy minerals being embedded in the red clay which cements the larger blocks.
Since the heavy minerals of the cascalho in the river beds are more worn than those of the terraces, it is highly probable that they have been derived by the cutting down of the older river gravels represented by the terraces; and since in both deposits the heavy minerals are more abundant near the heads of the valleys in the plateau, it is also highly probable that both have really been derived from the plateau deposit.
In the latter, especially at Sao Joao da Chapada, the minerals accompanying the diamond are scarcely worn at all; in the terraces and the river beds they are more worn and more abundant; the terraces, therefore, are to be regarded as a first concentration of the plateau material by the old rivers; and the cascalho as a second concentration by the modern rivers.
The mining is carried on by negroes under the supervision of overseers; the cascalho is dug out in the dry season and removed to a higher level, and is afterwards washed out by hand in running water in shallow wooden basins (bateas).
In 1901 there were about 5000 negroes employed in the Bahia mines; methods were still primitive; the cascalho was dug out from the river beds or tunnelled out from the valley side, and washed once a week in sluices of running water, where it was turned over with the hoe, and finally washed in wooden basins and picked over by hand; sometimes also the diamantiferous material is scooped out of the bed of the shallow rivers by divers, and by men working under water in caissons.