Made up mostly of calcium carbonate.
"Joachimsthal" is the German for it. Note how it is just near the modern frontier between Germany and the Czech Republic.
en.wikipedia.org/w/index.php?title=Uranium&oldid=1243907294#Pre-discovery_use:
In the early 19th century, the world's only known sources of uranium ore were these mines.
Apparently the region was a silver mining center:
Starting in the late Middle Ages, pitchblende was extracted from the Habsburg silver mines in Joachimsthal, Bohemia (now Jáchymov in the Czech Republic), and was used as a coloring agent in the local glassmaking industry
Falun mine tour by Focus by Sohaib
. Source. Not English, but we can see the tour images at least. Articles by others on the same topic
Mineralogy studies the composition, structure and properties of minerals. A chemical analysis identifies which elements are present, while X-ray diffraction constrains their periodic atomic arrangement. These measurements answer different questions: substances with the same overall composition can have different crystal structures.
Microscopy and physical properties provide complementary evidence. Mineral assemblages and textures help reconstruct rock formation, deformation and fluid interaction. Identification must allow for solid solutions, in which composition varies within one structural family, and for alteration that changes only part of a grain.