Among these natural resources that were previously important in the Kosovo economy were minerals and metals such as: lignite, aluminum, chrome, magnesium, nickel, lead, zinc, and several different construction materials.[9] Kosovo was historically a mining district for the former Yugoslavia. The industrial complex Trepça in Mitrovica "was the largest mining operation in the former Yugoslavia".[2][3] During the time of Yugoslavia, Kosovo possessed 50% of all of Yugoslavia's nickel reserves, 36% of lignite, 48% of lead and zinc reserves, 47% of the magnesium reserves, and 32.4% of kaolinite reserves.[10] During this time, mining was Kosovo's growth engine; however, the sector faced a decline from the 1990s.[4] Even prior to Yugoslavia, the region's natural resources were explored and extracted. During the reign of King Milutin (mid 14th century) the mine in Novo Brdo was considered the richest mine with silver in the Balkans, while the other two mines in Kosovo after Artana were Trepça and Janjevo.[10] During the mid 14th and 15th century, Artana was the heart of the local economy, it was known as the city of silver and gold.