AlbaGuBrath
"Scotland Forever"
For the most part, this particular trope stems from old D&D armor restrictions that were implemented for the sake of game balance. Some people justify it with the thinking that elves take everything that they need from the nature immediately surrounding them; things like mining and smithing would take more industrial effort and create a greater disturbance in the landscape and their surroundings than they would want. Others say that plate / heavy armor in general restricts the flow or use of magic in some way or another, or that wearing leather and pelts brings druids closer to nature (or in the case of mages, wearing fibers imbued with the arcane might make their magic stronger) or that people that spend most of their time studying simply haven't cultivated the physical strength needed in order to wear heavier protection.
But this is not a hard-and-fast rule for all fantasy anyway.
Also, stone would be crazy heavy as far as armor goes, assuming you wanted to make a set that allowed for roughly the same amount of protection as metal.
Well, actually, I meant stone knives and such. I don't think it was mentioned specifically, but if the restriction applies to tools or weapons as well, stone seems like the next option when wood wouldn't be appropriate.
But yeah, that makes sense. That first explanation was kinda what I was thinking of, and since we were playing in a world of the GM's making, we just went with that in the absence of another explanation. Thanks!