[churl]

If you forge 1 clam of pearls per turn (with a hammer), it takes 29 turns to "break even": the number of gems put in (29 clams at 14 gems each == 406) equals the number of pearls output (0+1+2+3+... for 29 turns == 406 pearls). If you need gems output the break even isn't for 57 turns! (1596 pearls output, which alchemizes to the 798 gems input)

For blood stones (forged with hammer), you break even on earth gems at turn 15 (105 gems), and blood at turn 23 (253 slaves becomes 253 earth gems).

For fever fetishes, assuming you place them on units with 10hp and produce 9 fire gems before switching to another unit, (and forge with a hammer), the break-even is around turn 16, using 96 gems (assuming some amortization of the gems output).

So, fever fetishes are by far the most efficient gem-producing artifact.. although they're also require the most micro-management.