The stones, averaging ten to a carat, are found in the river gravel or in alluvial deposits.
In diamond weighing the grain = ± of the carat, = 7925 of the troy grain.
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.
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.
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.