As suggested, level Gsm.

Level mining with raw Jewels, polish them with Gsm.
A raw will sell for 1 gil. A polished will sell for 15+

Be warned the shards out number the raw 3:1 and 5:1 so be ready to farm lots and lots and lots of shards.

The trouble with this is, the shard farming will burn you out that much faster and you will get so sick of mining shard you won't want to level out the class.