Many historically valuable buildings are in extreme danger due to permanently growing deterioration. For centuries many of these natural stones have been exposed to the weather. Permanently changing conditions of humidity, aridity, frost and wind cause chemical and mechanical processes which influence the properties of the microstructure of the natural stones. The properties may change until damages are macroscopically visible. In addition to environmental stresses caused by weather conditions, increasing stresses due to pollution like SO2, NOx, CO2 etc. act on the stone in recent years. Some of the deterioration effects are accelerated by these additional stresses. The investigation of causes and mechanisms of the destructive processes is of great importance for the development of preservation- and protection-methods which will counteract further decay of the monuments. Up to now characteristic mechanical data of natural stones, especially related to weathering, have barely been investigated. Moreover, the strong inhomogenities of natural stones restrict the application of some parameters like elasticity module for their description. Within the scope of a joint project on monument preservation the mechanical properties of a variety of natural stones shall be investigated. In addition to informations obtained by established methods of material testing, position- and time-resolved data on deformation processes up to crack development are required. The latter suggests the introduction of optical methods which allow the observation of even small deformations on the whole stone surface during a test cycle. Thus a quick interpretation of the observed process is possible.