The Blue John Cavern is one of the four show caves in Castleton, Derbyshire, England.

The cavern takes its name from the semi-precious mineral Blue John, which is still mined in small amounts outside the tourist season and made locally into jewellery.

The miners who work the remaining seams are also the guides for underground public tours.

In 1865, Blue John Cavern was the site of the first use of magnesium to light a photograph underground.

[2] In the UK Blue John, or "Derbyshire Spar", is found only in Blue John Cavern and the nearby Treak Cliff Cavern.