How is it free after the initial wish? You get the 20S from the site and 25S from the wish, you still need 55S gems for a continued wish the next turn and I consider a regular game to have let's say 20S gems income on average.
If you save all gems from forging/casting you still lack gems to cast a gem wsh. While it may cost 80 to cast in theory you need the full 100 and keep the 20 as a deposit.

(I think)
If you're alchemizing gems (which you're probably fire cause they're worthless), it makes for an even more expensive gem gen with less initial profit (but I guess all gem gens have opportunistic cost so ok), which given, undoubtedly will more than compensate you after a dozen or so turn...
But my point is, it takes a while to really kick it off and make some pearls since you won't be making the full 20S per turn from the start, or you will at the expense of losing other gem types to alchemising.
As the game progress you might be casting it multiple times per turns and getting more out of the site as you go, also adding another wisher faster and faster every time for more profit and so on...
So I'm just saying, you can't really consider that game breaking cause it takes a lot of time and risk getting to the point where you gain a substantial amount that will affect the game in such a manner.