Portoro, the black and gold Italian “marble”

The “Portoro”, called also “Mischio giallo e nero”, “Portovenere marble”, Black and Gold”, is one of the most famous and expansive “black marbles” for its characteristics yellow gold veins on a black background. It is a limestone of Rhaetian-Hettangian age belonging to the Tuscan Nappe. It was used since Roman times, particularly in the city of Luni. Since the Middle Ages its use is widespread in Genoa, for the construction of monuments, cathedral, villas. At the end of the XIX century its use has spread abroad, especially in England, where it was used for fireplaces, coverings, plinths and panels for furniture. It was extracted in several quarries located near la Spezia, precisely on the promontory of Portovenere, in Palmaria Island and Tino Island but at present only one quarry is active. The low volume production of this “marble” is now challenged by similar commercial marbles, cheaper and with huge production, from abroad. This material, exposed to the weathering, tends to fade with time losing the look of his golden streaks and spots that determine all the aesthetic appeal. In the paper, this chromatic alteration will be investigated in order to define the guidelines for the most suitable use of this stone and for eventual restoration works of recent as well as historical monuments. Moreover the knowledge and awareness of the characteristics of this precious stone material may help in recognizing and maintaining the value of Italian Portoro.