Based upon studies of natural quartz and upon laboratory experiments on mechanical and chemical destruction of quartz, it was established that the statistical density characteristics of detrital quartz change during sedimentary processes. Grains with defective structure, gas-liquid inclusions, inclusions of organic matter and other lighter minerals have lesser densities, whereas grains with inclusions of heavier minerals have larger densities than those of optically pure grains. The susceptibility to destruction is less for optically pure grains than for grains with lower or higher density. This is responsible for the increasing maturity of quartz sand during weathering and transport as indicated by the increasing percentage of optically pure grains. Mathematical modelling of the dynamics of this process was carried out.