So the annual production of this region, which was estimated for the years
preceding 1870 at 3,000 oitavas (about 52,000 carats), is now scarcely
500.
The rivers in the environs of Diamantina rim at the bottom of deep and
narrow gorges that have been scooped out to depths of 300 or 400 meters
through the denuded plateau in whose center stands the city of Diamantina.
In the bed of these rivers, in places where they have not yet been worked,
there may be found, underneath a stratum of modern sand, another of rocks,
and finally a diamondiferous deposit of rounded pebbles, mixed with sand.
This gravel, which is characterized in the first place by the fact that
all its elements are rounded, and next by the presence of a large number
of minerals (among which the most important are all the oxides of
titanium, different oxides of iron, tourmaline, and a whole series of
hydrated phosphates of complex composition), is called in the language of
the country _cascalho_. It is the matrix of the diamond, and the latter is
extracted from it by washing. It is arranged in roundish masses upon the
beds of the rivers, and is met with at depths ranging from a few
decimeters up to 25 and 30 meters.
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