What can blue john be used for?

Question

ACoating steel, particularly gun barrels, to protect from rust

BWhitening clothes being washed

CStoring Johnnie Walker blue label whisky

DJewellery, drinking vessels, vases

Select from the options above.

Correct Option: 4  

Explanation:
Jewellery, drinking vessels, vases
Blue John is a form of fluorite banded in purple-blue or a yellowish colour, a hard semi-precious mineral. It is found near Castleton, Derbyshire, UK, where it has been mined since at least the 18th century, and apart from a similar banded fluorite recently found in China it is the only source of this particular banded fluorite. Blue-john is also a term sometimes used for skim milk or milk which is beginning to sour.