The quartzite of Tienen is the most important building stone of the Romanesque and early Gothic times in the Belgian Gete-river basin. It is characterized by its great hardness, high SiO₂ content, predominant quartzitic texture, and the absence of glauconite and chlorite. The quartzite of Tienen frequently contains plant roots and sometimes fragments of silicified wood. These are less common in the quartzitic building stones of the same geological age (late Paleocene-early Eocene), found in the Belgian Halnaut province and northern France (Amiens, Douai). The petrographic characteristics of the quartzites of Wommersom and Avebury (South England) used in prehistoric times and closely related to the quartzite of Tienen are discussed. -- AATA