Blue John (A)
. A petrefaction of blue fluor-spar, found in the Blue John mine of Tre Cliff, Derbyshire; and so called to distinguish it from the Black Jack, an ore of zinc. Called John from John Kirk, a miner, who first noticed it.
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Entry taken from
Dictionary of Phrase and Fable,
edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.