The stones, averaging ten to a carat, are found in the river gravel or in alluvial deposits.
African mines the diamonds are not only crystals of various weights from fractions of a carat to 150 carats, but also occur as microscopic crystals disseminated through the blue ground.
Basing the calculation on the figures for the ten years 1896-1905, the average annual production is slightly over two and a half million carats, of the average annual value of £4,250,000, the average price per carat being £1: 13: 3.
In spite of this, however, the average yield in the profitable mines is only from 0.2 carat to o 6 carat per load of 1600 lb, or on an average about IIgrs.
The annual output of diamonds from the De Beers mines was valued in 1906 at nearly £5,000,00o; the value per carat ranging from about 35s.