Despite the general poor returns in recent years, mining retains a certain importance in Andalusia. Andalusia produces half of Spain’s mining product by value. Of Andalusia’s production, roughly half comes from the province of Huelva. Mining for precious metals at Minas de Riotinto in Huelva (see Rio Tinto Group) dates back to pre-Roman times; the mines were abandoned in the Middle Ages and rediscovered in 1556. Other mining activity is coal mining in the Guadiato valley in the province of Córdoba; various metals at Aznalcóllar in the province of Seville, and iron at Alquife in the province of Granada. In addition, limestone, clay, and other materials used in construction are well distributed throughout Andalusia.