blue John

noun3

1. •
   A semiprecious mineral mined in Derbyshire, a form of fluorite with bands of a purple-blue or yellowish colour.
2. •
   (US) Skimmed milk.

   • Lee went up front to his icebox and brought back a jar of milk. The cream had been skimmed from it. I said, “This tastes like blue-john to me.” — 1968
3. •
   (no longer used) Afterwort, partially fermented wort.