Black Slate locations for photography

[brotherharry]

Coming for a hoon to North Wales at the weekend and want to track down some nice black/dark rock locations as backdrop that will offset my green snot-rocket.
Any locals care to recommend roads/spots that might be suitable?

[brotherharry]

Not much luck on hacking a quarry, but Horsehoe had a few options:




Also discovered the B4391 running off the A4212 to Ffestiniog:


Good times