A survey of the counting characteristics of thirty particle-counting diamonds has been made. It was found that the diamonds can be classified on the basis of their counting behaviour in the dark and during subsequent illumination. The dark characteristics and the counting response to illumination are explained in terms of the trapping and recombination of charge carriers via three levels in the forbidden energy gap. It is shown formally that the three-level model predicts the empirical classification of the diamonds on the basis of varying concentrations of the defects that make the diamonds be of type Ia. The asymmetry of the counting property is due to inhomogeneity of the defects and the counting property can be put into correspondence with the purity of a diamond, the purest diamonds being the best counters.