Mana farmed is reduced over time so I'm not sure how lucrative this may be, these corrupt shards were a giant exploit on release and this is the reason the mana gain was reduced over time. By using an amped crystal you could get insane gem grades extremely fast (the poolbound gem has been nerfed since as well i believe) and was a giant snowball effect.
The question is now - How long do you farm this before it's not worth the return anymore.
zzxxzz, you seem to be good with GC:CS mechanics. What has got influence on mana gained from mana shards (gem damage, firing speed, maybye specials?) from specials multiple damage only, or other too?
No idea because I generally ignore mana shards, but I'm pretty sure it's speed and damage only. That means yellow gems are much better at farming than other gems.
I can tell you that shooting a gem does not count as a "hit," so doesn't do anything for black gems...makes me think that orange gems wouldn't get extra mana either then.
For corrupted mana shards: use beam. It does damage per frame (I think) but always does 1 damage at least, so a gem with 1-3 damage/shot will get much more mana from beam than it otherwise would.