These unusual iridescent galena specimens are exhalative formations that come from a no longer operational coal mine with a burning mine dump. The galena crystals form at a temperature of more than 600°C in the mine dump. They are slightly radioactive (not nearly enough to cause harm). The specimens come with a handwritten label from a dealer based in the Czech Republic.
http://tw.strahlen.org/haldenbrand/burningdumps_radvanice.html
https://www.sciencedirect.com/science/article/pii/B9780444595096000053
