“ Mining in a certain area turns all exposed skin (maybe just parts that are actively disturbing the chemicals that cause the reaction) of the those doing the mining to a dark blue color. Will wear off taking as many years/days spent in the mine.”
“ Orcs record their deeds through symbolic mutilation, scarring and tattoo. Perhaps creating the medium of skin as art, which humans followed later. Perhaps tattoos remind the elves of orcs and make them angry...”
“ A city is being plagued by a swarm of winged mice. The vermin can get anywhere, and they can easily avoid their normal predators by simply flying away from them.”