Decin mine which operated between 1955 and 1994 contains over 20 kilometres of mine galleries. The mineral being mined was Fluorite - the mineral form of Calcium Fluoride used mainly as a flux in smelting. Other uses include the production of hydrofluoric acid and the manufacture of high quality lenses, especially for use in the ultraviolet range. A purple form of the mineral, known as ‘Blue John’ is well known in the UK, being mined near Castleton, and used for jewellery.
During the operation of the mine a number of pseudokarst cavities were discovered underground. Pseudokarst refers to underground cavities found in a bedrock that is not soluble – in this case siliceous sandstone. These cavities are coated in almost pure fluorite with an attractive crystal structure. Since 2001 the site has been managed by the Cave Administration of the Czech Republic and access is stricly controlled.