Correct block size assessment is the most important stage for rock quarry management. Although volumetric joint count (J(v)) and weighted joint density (wJd) were proposed for this purpose, simple prediction method for these indices is not encountered in literature. Due to the fact that some rock masses such as marbles contain less discontinuity, collection of representative amount of data from in situ line surveys for statistical assessments is highly difficult. For this reason, the main targets of the present paper are to apply photoanalysis approach for collecting additional discontinuity data and to obtain some simple statistical and fuzzy models for predicting weighted joint density to evaluate block size in engineering practice for marbles around Supren (Eskisehir, Turkey). In addition, a new and simple approach to predict volumetric decrease caused by chemical weathering is introduced. For these purposes, extensive field and photoanalysis studies were performed and the data obtained from both field and photoanalysis studies were assessed by regression and fuzzy approaches. The results revealed that the prediction performance of the fuzzy inference system is higher than that of the regression equation. (c) 2006 Elsevier B.V. All rights reserved.