Knowledge of the durability of rocks, regardless of their size or shape, is essential for determining the safety and lifetime of structures and monuments. Hydraulic structures in particular require a high probability of survival of at least 100 years and monuments closer to a millennium. The degradation of a rock is often caused by clay minerals which are present in the rock from its origin and/or which have been formed in the rock in its geological history on account of the less stable rock constituents. In their fresh state during quarrying and mining it is difficult to recognize tendencies toward degradation and decay in rocks with time. For the selection of high quality construction materials (basaltic rocks) a series of new testing techniques has been developed in which the focus is on the role of clay minerals. This paper deals with occurrences, neoformation and behaviour of clay minerals which contribute to an early recognition of tendencies of rock decay.