Cylindrical rock columns in the Great Mosque in Diyarbakir Old City are investigated for their lithology and architecture. The columns belong to four different rock types, namely pre-Tertiary meta-ophiolites, Eocene limestones, Miocene limestones and Plio-Quaternary basalts. The columns from the first three lithologies are reused during the construction and/or renovation of the Great Mosque. Thin sections prepared from 18 columns of this building confirm they are derived from the rocks exposed in the region. The reuse of the columns is approved by inconsistent column lengths, multi-segment column shafts, lithologically mixed columns and presence of thin wedges at the bottom of column bases. These columns are not used to support the main body but rather either for partial support or only for decorative reasons.