Heh. That brings to mind one of my game-breakers: ever-smoking bottle + cloudkill. Place at entrance to Underdark, become a god.
It's cool. Think of us as providing a service. We're pre-breaking your game, so your PCs don't get a chance to! The first rule of DMing, a corollary of The Law of Unintended Consequences, is that PCs will do two things you anticipated, and eight that you didn't. That's why it's good you brought this to us: so that we can go over the eight you didn't, and you can anticipate, plan, and refine. We've already seen good ideas like this, and how unanticipated consequences cause DMs to run out of "Yes, but"s and fall back on "NO". This gets feelings hurt and can destroy games.
In this case, it's that PCs will find a way to exploit any source of power (the abstract, not electricity) that you give to them. The stones are an obvious source of power. The upside is that this is going to make them coveted, by anyone and everyone. One way to head off some of their wacky hijinks is to already have some of this stuff pre-generated. The city already has a water stone-powered mill, thank you very much, and a hammer mill to boot. It's even got a heat stone powered forge -- tightly controlled by the blacksmith's guild, of course -- and that's the best place to get all of your top-quality gear (at a top quality price). They start coming up with really crazy innovations, that's where you can step in and say, "Whoa there Archimedes. And your character can come up with this shit HOW, again?"