I've discovered a problem with some of my gold.

[swizz]

oops, I was under the impression that he actually mined the material himself
could well be a lark then

[rivraton]

If it turned black then more then likely it is not gold.
Gold don't do that.

Unless it in in contact with manganeese:
BLACK GOLD
a. A slang American term referring to crude oil.
b. Syn: Maldonite
c. Placer gold coated with a black or dark-brown substance (such as a film of manganese oxide) so that the yellow color is not visible until the coating is removed. AGI
Source: Dictionary of Mining, Mineral, and Related Terms