To my knowledge,
Elves get more of the rare resources, so if a human would be mining on a valorite spot, he would get 6 valorite and 20 iron, while the elf would get 10 valorite and 12 for example. Elves get lesser resources then humans overall, althou the difference isnt really big. Gems has nothing to do with elves or human. I always use humans just cos they can carry more and dont waste that much time recalling around.