Tournai Stone is a specific type of limestone renowned for its hardness, compactness, and a distinctive dark bluish-gray hue. This stone is extracted in the vicinity of Tournai and serves as a notable source of blue stone, extending across a significant part of Wallonia and neighboring regions of France and Germany. The quarries in the Tournaisis region yield various varieties of Tournai Stone, each differentiated based on the geological layers they originate from.

Formed through marine sedimentation during the Paleozoic era, Tournai Stone has a fascinating geological history. Its development spans multiple stages, with a notable layer being the Tournaisian of the Lower Carboniferous. This layer, from which the stone draws its name, is associated with the city of Tournai, where the stratotype—a standard reference section for the layer—is located.

Tournai Stone's robust nature, coupled with its unique coloration, makes it a sought-after material for various construction and architectural applications. The stone's prevalence in Wallonia and its surrounding regions has contributed to its historical significance and enduring popularity in the realm of natural stone materials.