Three different types of block sizes, namely, bars, plates, and prisms of various dimensions, are related to the classical rock quality designation (RQD) index by means of the volumetric joint count definition. Statistical models are derived analytically by assuming that three sets of fractures at different directions occur according to a negative exponential distribution function of the intact lengths. This distribution function has one model parameter, which is the average number of discontinuties related to the block volume and the volumetric joint count. Necessary charts are presented, which help to identify the representative block size within a rock mass provided that the volumetric joint count and average number of discontinuties are estimated from the field surveys. Furthermore, the global volumetric joint count is defined and related analytically to the nonhomogeneity coefficients in different directions. Finally, relevant analytical expressions are derived and presented in the form of charts for the relationships between RQD, block volume, and volumetric joint count. Accordingly, excellent, good, fair, poor, and very poor rock quality classifications are delineated on these graphs. Finally, the application of the methodology developed herein is presented for a set of discontinuity survey on igneous rocks in the Kingdom of Saudi Arabia.