The Connemara Marble of Ireland is a distinctive green decorative stone used in world architecture. These stones were imported and widely used in the Carnegie Institute Extension built in 1907 which is acknowledged as one of the America's finest Gilded Age Beaux-Arts structures. This report documents the use, quantities, and locations of Connemara Marble in the Carnegie Institute and through a review of historic primary source documents, secondary sources articles, on-site inspections, sampling, testing, and microscopic analysis has determined the stone's mineralogy and petrology, color types, and the specific location and identify of the actual quarry site and the lithostratigraphy from which these materials originated. In the 1970's the Carnegie Institute was listed in the National Register of Historic Places by the United States Department of the Interior and named a Historic Landmark by the Pittsburgh History Landmarks Foundation. In 2019, the Connemara Marble was proposed as a Global Heritage Stone Resource to the International Union of Geological Sciences. The Carnegie Institute incorporates 32 varieties of igneous, metamorphic, and sedimentary rocks types from Algeria, Croatia, France, Greece, Italy, Ireland (i.e., the Connemara Marble), and the United States of America. The building's architects utilized four-color types of Connemara Marble in 13 prominent interior spaces of the building. The Connemara Marble formed from Precambrian Dalradian carbonates during the lower Ordovician Grampian orogeny, some 470 million years ago in western Ireland. Approximately, 366 square meters of floor tiles and wall inlays were fabricated out of some 60 metric tons of Connemara Marble blocks extracted from the Streamstown quarry near Clifden, County Galway.