Coerthas is good for gems, but to get all six elemental types of gems you have to visit all four areas. Each of the areas has different types of gems. Mythril can be retrieved in any of the four grade 5 areas (and at a perhaps lower rate in grade 4). The only issue I have with Coerthas mining is that there are only four mining points active instead of five, so it occasionally means a bit more legwork.

On the plus side, Dragonhead has an NPC merchant so you sell off the sluff gathered materials instead of dropping it on the ground.

Best of luck!