In northern Belgium siliceous rocks have been used as local building material in former centuries. The quartzite of Tienen can be distinguished from other Belgian natural building stones by its great hardness, its equigranular quartzitic texture and by the absence of glauconite and chlorite minerals. It frequently contains silicified plant roots. The tuffeau of Lincent is a porous opaline limestone, characterized by a low density, by the presence of numerous dissolved sponge spicules and by a high garnet content in the heavy-mineral fraction. The ironsandstone of Diest and the Holocene bong-iron ores of North-East Belgium are ironrich materials displaying a different texture and showing a different mineralogical composition. Flint can be distinguished from other silica-rich materials by its smaller grain-size. The high glauconite content is the distinguishing and identifying feature of the siliceous quartz sandstone known as veldsteen .