This reef was then raised above sea level by an uplift in the Earth's crust and was subsequently eroded, forming a boulder bed on the slopes.

Major earth movements at the end of the Carboniferous period reversed this subsidence, and the rock layers were raised to form an anticline, the top of which was eroded away.

[2] Trace elements from buried sediments, percolating rainwater and trapped seawater meant inorganic ions were present within the porous rock, among them chlorine, fluorine, sulfate, calcium, barium, lead, zinc, and even uranium.

The exact cause of the coloration is disputed, but it seems that the formation of Blue John requires a rare combination of porous limestone containing a little hydrocarbon, small traces of radioactive elements, an impervious layer to prevent the mineral fluids and hydrocarbons from escaping, a high concentration of fluorine ions, and variations in conditions during the crystallization process to form the banded colours.

[5] The Old Series is thought to have been discovered in about 1745–50 by miners seeking lead ore, and is known to have been a source of Blue John by the 1750s, at which time it was known as Millers Mine.

This mine is the grand depository of the amethystine or topaziue fluor spor, locally called "Blue John," to distinguish it from "Black Jack," or zinc ore.

But the most beautiful of all the clambers is that called the "Crystallised Cavern," a large dome-shaped cavity, the height of which is estimated at 100 feet, and whose sides are adorned with numerous stalactites, that sparkle like stars when it is lighted up.

Demand for fluorspar increased during World War I and much ornamental Blue John was extracted for use as flux in blast furnaces and in the chemical industry.

[2] Commercial mining ended shortly after 1926 but in the 1930s miners returning to retrieve stored Blue John discovered a new, easier route into the New Series.

Some of the formations have also been given names, according to their perceived resemblance to other objects, such as the Stork, which forms part of the cavern's corporate logo,[6] and the adjacent Ram's Head, both within the Dream Cave.

The lost vein, concealed by wooden battens, a piece of carpet, and a layer of clay, was rediscovered in 2013 by John Turner, Peter Harrison's grandson.

[10] In 2015, another vein of Blue John was discovered close to the tourist route by a miner experimenting with a new method of exploration using a specialist chainsaw.

[11] In April 2014, after 18 months of hard digging, 2 local cavers - Mark Cope and Martin Barnsdall discovered an extension deep underneath 'Dome of St Pauls'.

At the end is a small vug of Blue John and Calcite, then underneatha tight squeeze leads to the top of a steeply sloping passage - '2014 Passage', off to the west side is a highly decorated chamber 'Wonder Cave', and small tight squeeze leads to a tiny chamber 'Emilys Cave' (after Marks then newly born daughter).