A field study was carried out in the Forum and Palatine areas (Rome) and volcanic areas of that province, in order to identify monuments which used volcanic materials as structural or decorative elements. There were six main types of volcanic stone identified. A complete set of data on their petrography, uses, location of ancient quarries, and type of alteration was collected. The chemistry of the alteration and its products is described. Results of conservation treatment tests with two consolidants, both in situ and in the laboratory are given for the six types of stone under study. The treatment efficiency was assessed by means of the water absorption index. Results show that the same consolidating treatment applied once apparently provides better protection than when applied repeatedly.