This Elementium Ore farming guide will show you the best places for mining Elementium Ore. It's recommended to have Mining skill 425 before you start farming, so you won't get any ore fragments.
I must also mention that Pyrite Deposits are rare spawns of Elementium Deposits in Uldum and Twilight Highlands.
Twilight Highlands the best zone for farming Elementium Ore.
Uldum is a great alternative if someone is already farming at Twilight Highlands.