The transitional siliceous rocks from the Belchatow lignite deposit belong to the deposits with heterogeneous petrographic composition. The research allows us to identify among others, opoka-rocks and gaizes. The mineralogical-chemical analysis proves that the main ingredients of the studied rocks commonly used as building material are minerals of the SiO₂ group. Laboratory tests show that the nature of siliceous mineral phases has several effects on the geomechanical parameters of the studied transitional rocks. They are a reduction in water content and rock porosity, which leads to the transition of opal type A to opal type crystobalit and trydymit and then to quartz or microquartz. Their density and strength parameters are increased.