Ireland's mineral resources are divided into three main categories: non-metals and carbonaceous deposits. The principal component of the non-metals categoey is construction materials, including building stone, aggregates, cement, clay and gypsum. Other important minerals in this category are magnesite and barytes. Metals are considered in the context of three different formative geological environments: Carboniferous Limestone, Devonian and Lower Palaeozoic rocks. The first of these is by far the most important, involving the major deposits at Navan, Tynagh and Silvermines, the exhausted deposit at Gordrum and a number of other minor deposits. The only working deposit remaining from either of the other two types of environment is Avoca, Carbonaceous deposits are subdivided into: peat, of which Ireland is the world's second producer; coal, which is of little economic significance; and oil and gas, some of which have already been discovered. Further oil/gas discoveries are anticipated from a major exploration programme beginning in 1976.