text: The build-up of calc-sinter material in the Eifel Aqueduct was commercially exploited in the 11th and 12th centuries. With deposits up to  thick, the material was cut into vertical columns of polished brown rock with impressive layered patterns, which made it much in demand by cathedral builders across large parts of central Europe and beyond. In England it was used to provide polychromy, contrasting with the pale limestone favoured by Norman English Cathedrals. The stone was for many years known as Onyx Marble despite being very obviously neither onyx nor marble. Those studying the stonework at Canterbury Cathedral were unaware of its origins in the aqueduct until 2011. Such large-scale use as the cloisters around a cathedral quadrangle needed many hundreds of columns, which must have been supplied by a well-organised extraction and transport operation. The Eifel deposits, have also been identified at Rochester and in the now lost Romanesque cloister at Norwich as well as the Infirmary Cloister, Chapter House windows, Anselm Chapel door and the Treasury gateway at Canterbury.