If you're looking for them for completion purposes and already have high Dojo stats with Farfields gear, using Caelo with Resonant Prong will be easier.
If you're still progressing through the game, I'd recommend not prioritizing the Diamond Pickaxe blueprint since there are stronger pickaxes in Farfields.
Also, the Cobalt Pickaxe blueprint drops in Farfields, not in the Magma layer. Hope this helps!