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Tolkien's "dark elves" weren't black-skinned, nor did they live underground, worship spiders or take slaves. They were called "dark" elves (Moriquendi) because they never saw any light before the sun and moon (i.e. the sacred light of the Two Trees).
Tolkien had a big thing about light. And its opposite, darkness.
RuneQuest made the silver piece the standard of trade - and that was in 1978!
Because that's what it was back in medieval times. No idea why medieval history freak Gygax dropped the ball on that one. Gold was only used by merchants to settle accounts amongst themselves, or by nobles to hoard wealth.