I like the card; makes pretty good sense in earth, too.
gemfinder: +1 quanta each turn, including turn 1
worker (assuming upped tower and worker)
turn 1: -1 investment (casting cost) = -1
turn 2: -1 activation cost, +1 for upped tower entering play, +1 for that tower at EOT. = +1
turn 3: -1 activation cost, +1 for upped tower entering play, +2 for this tower and turn 2 tower = +2
turn 4: -1 activation cost, +1 for upped tower entering play, +3 for this tower and turns 2/3 tower = 3
So in the upped case, it takes 4 turns to beat out a gemfinder. Unupped towers/worker it's worse; too lazy to figure it out. However, the worker adds flexibility in duo decks, or especially rainbow decks. (quantum towers breeding = h4x :p)
I see no reason why it couldn't work on pendulums; that is both reasonably powered and makes sense. (or even tourney mark piles) It's overpowered on quantum towers though.