In the territory of the Slanske Vrchy mountains, between the towns of Zlata Bana and Cervenica, there are famous Slovak opal mines.

An excerpt from the article 37 facts about Slovakia

These mines are now a valuable monument in Slovakia. They had their best time in the years 1845-1880 when the tenant of the mine was the Viennese jeweler Salomon Goldschmidt, and after him, his descendants. This family exported the obtained opals to world markets and gained them worldwide fame. The largest opal in the world is Harlekyn, found in 1775 in the bed of the Olsavka stream (2970 carats). It is an exhibit of the nature museum in Vienna - its price is estimated at half a million USD. The Burning of Troy opal is part of the French Crown Treasure. In 1889, a nest of opals was discovered in the Viliam adit, which was named Gisela Chapel. This nest produced 200 kg of these precious gemstones.