Szakáll Sándor - Jánosi Melinda: Minerals of Hungary (Topographia Mineralogica Hungariae 4. Miskolc, 1996)

Splendid honey-yellow and reddish brown opals (honey- and fire-opal) were collected as long ago as the 18th century from narrow shafts in the region. Documentary evidence prove that these opals were made into gems, but that it was quickly discovered that they cracked rather easily, hence, were not durable as gemstones. Some of these specimens were accompanied by small grains of beautifully coloured precious opal. Miocene pyroclastic rocks, widespread in these mountains, have rather restricted mineral assemblages, but contain a number of industrially useful minerals. Rich clay deposits near Szerencs, Mád, Rátka, Füzérradvány, Bodrogszegi are associated with the pyroclastic