W. E. Sharp
A log-normal distribution of alluvial diamonds with an economic cutoff
Economic Geology and the Bulletin of the Society of Economic Geologists (May 1976), 71(3):648-655

Abstract:
The total alluvial diamond output from the Griqualand West area of the Cape Province of South Africa for the period 1917 to 1965 is a log-normal distribution. The estimated geometric mean of the deposits is 4,070 carats, and the corresponding parameters of the transformed data have a mean and standard deviation of 3.61 and 0.73, respectively. The observed distribution is censored at the low-production end because these deposits were uneconomical to operate. The break in the data suggest that deposits containing less than 2,000 carats, having a worth of R20,000 (U.S. $30,,000), were too small or too dispersed to sustain a viable operation, while any deposit which in the normal prospecting period yielded less than 200 carats, having a worth of R2,300 (U.S.$3,200), was abandoned.