Denmark's industrialized market economy depended on imported raw materials, its mineral resources were mainly fossil fuels in the North Sea, and the nonfuel minerals industry included mining and quarrying of chalk, clays, diatomite, limestone (agricultural and industrial), and sand and gravel (onshore and offshore). The industrial minerals sector was particularly active. There were some 90 pits in Denmark from which clay was mined; this material was used primarily by the cement, brickmaking, and ceramic tile industries. The production of sand, gravel, and crushed stone has become more important in recent years, not only in meeting domestic demand, but also as an export to Germany and other Scandinavian countries. Kaolin, found on the island of Bornholm, was used mostly for coarse earthenware, furnace linings, and as filler for paper; production was 2,000 tons in 2001, down from 3,000 in 1997. There were important limestone, chalk, and marl deposits in Jutland. Chalk production totaled 410,000 tons in 2001. Limonite (bog ore) was extracted for gas purification and pig iron production. Large deposits of salt were discovered in Jutland in 1966; in 2001, 605,000 tons were mined. The country also produced fire clay, extracted moler, lime (hydrated and quicklime), nitrogen, peat, crude phosphates, dimension stone (mostly granite), and sulfur. According to the constitution, subsurface resources belonged to the nation, and concessions to exploit them required parliamentary approval.