Diamonds can be found as rare drops from Ore, so pick up any ore you find. The best locations for ore are the Great Pyramid and Tomb of Storms in Krokotopia, the Cave of Solitude in Mooshu, and the Athenium in Dragonspyre.
You can also harvest diamonds from Fickle Pickles, Frozen Fly Traps, and Burning Snap Dragons.
Fickle pickles are available for gold in Golem court. Just buy a couple because they'll drop more seeds at harvest and soon enough you'll be overrun. The extras sell for $480 each at the Bazaar.
You can either hunt down ore reagants for a chance at getting a diamond, or you can purchase a "transmute diamond" spell from several crafting NPCs in the spiral. You will still need to collect ore to transmute, but it's much f aster than hoping for diamonds in an ore reagant. Felicia Worthington in Marleybone sells the spell. If you do not have access to this area yet, ask someone to take you there.
Happy questing! Hope to see you out there! ~Erica Lifepetal 120 ~Sarai Fire 73 ~Jenna 86