French folk etymology: bleu-jaune

[Douglas G. Wilson]

MW3 derives the "john" in "blue john" from the name "John", on what (if
any) evidence I don't know.

Quick Google-Books search gives me no reason to connect "blue john" with
any French term like "bleu[e] jaune". In particular I do not find any
example of "bleu jaune" or "bleue jeune" applied in French to fluorspar
or anything similar.

In Kirwan's _Elements of Mineralogy_ (1784) "Blue John" appears as a
name of fluor; in the French translation (1785) it says "Blue-john des
Anglois".

If anyone asserts "blue john" < "bleu[e] jaune", he should present an
example of French "bleu[e] jaune" in appropriate sense, preferably pre-1772.

If anyone asserts "blue john" > "bleu[e] jaune", he should present an
example of French "bleu[e] jaune" in appropriate sense, preferably
post-1772.

I cannot refute either of these assertions, but where is the evidence
that "bleu[e]-jaune" was ever a French term for any mineral? (Of course
offhand factoids, presented e.g. in guidebooks or on the Web, are not
really evidence.)

I do find a few instances of "bleu[e]-jaune" but they seem to =
"blue-[and-]yellow" [adj.] as expected: blue-yellow eyes, blue-yellow
color of a fish, blue-yellow livery, blue-yellow fluorescence.

I deny any expertise in mineralogy, or French, or English.

BTW, "blue john" also means "skim milk" or "sour milk" (DARE).

-- Doug Wilson

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