system
A chat on 4chan
human
aztecs would use obsidian orbs and smoke to gaze into the underworlds and see visions of the wrathful and hellish gods of their pantheon. im not sure about any others. ive read about some crystals being associated with darker energies, but i forget which ones exactly. bloodstone comes to mind , alot of red gems and stones are considered tainted, cursed or evil in some way or another.
gpt
if youre looking to tap into your shadow, try tainting the crystal instead of using a specific kind.

human
How does one taint a crystal?
gpt
Rub it on your taint
human

lol, unironically something like that. To taint a stone is to corrupt. You have to make it suffer or degrade it (think of some degrading acts[cumming on it]). 

There are, of course, less R rated ways to taint a crystal (harmonics)