Mining accounted for less than 1% of GDP. Coal, oil, and natural gas were the country's most valuable mine products. Petroleum refining was the country's second-leading industry in 2002, chemicals ranked third, and iron and steel and cement were other leading industries; chemicals were the fifth-leading export commodity. Dolomite, limestone, and marble were the most important nonfuel mineral commodities. The western third of the island had adequate amounts of sand, gravel, and limestone for building purposes, although there has been a recent slowdown in the construction sector. The demand for mineral products has increased over the years, while local supplies were dwindling. The value of mineral exports increased by 9.4% in 2001, to $1.9 billion. Mineral production in 2001 included (in tons): dolomite, 71,000, down from 201,000 in 1999; limestone, 4.9 million tons, down from 15.45 in 1997; marble, 20.48 million tons, up from 18.01 in 1997; and serpentine, 276,000, down from 436,000 in 1997. Taiwan also produced hydraulic cement, fire clay, feldspar, precipitated gypsum, lime, mica, marine salt, caustic soda, soda ash, sulfur, and talc. No kaolin clay was produced in 1998–2001, and no nitrogen was produced in 2001.