Mining's contribution to Cape Verde's economy was minimal, and the geological potential of the islands remained largely unexplored. Pozzolana (a volcanic rock used in pulverized form in the manufacture of hydraulic cement) from four mines on Santo Antão and salt were the only minerals exploited commercially, salt being a leading industry. In the mid-1990s, about 4,000 tons of salt and 5,000 tons of pozzolana were quarried. Sal and Boa Vista had sea-salt refineries and deposits of calcareous rocks, used in paving, building ornaments, and tile production. There were also deposits of kaolin, clay, gypsum, and basalt.