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roundness index The average radius of curvature of the corners of a particle, divided by the radius of the maximum inscribed circle for a two-dimensional image of the particle, i.e. (Σr/N)/R, where Σr is the average radius of curvature at the corners, N is the number of corners, and R is the radius of the largest inscribed circle. In practice, it is used empirically and other techniques are also used. For example, a pebble may be compared with a set of standard silhouettes. The resulting index figure allows inferences to be made about the nature of the depositing process.