This is the set's Darksteel Ingot variant, implemented as something like a Fertile Ground. It's in the power sinks cycle as described on Venting Pipewalker, and effectively makes "[Torque]: Add two mana of almost any colour to your mana pool".

In initial playtesting this turned out far more fiddly than I'd hoped. I do like the synergies in this version, but it's just too brain-bending. Will probably convert it to something much more boring like ": Add 1 mana. // [Torque]: Untap this."

Hm. Other people seemed to have a problem because they tried to activate it and then later on in the turn use the land and had to remember; I couldn't see why not to normally use the abilities just before you tapped the land anyway, and then it's effectively "add blah extra".

OTOH, I completely and totally missed that it wasn't "the next time this land is tapped for mana". I hadn't realised this could get 1 extra mana normally and 2 more with torque. And if you have the "untap all other artifacts" connector, you can get two extra mana from tapping this twice, and three from torque. I'm not sure if Stewart realised this either, I think he was using this to get red to ping me with the land, but I can't remember exactly how he used it. I think remembering these triggers is too fiddly (certainly for common) but you may be able to fix it with tweaking the abilities.

Alternatively, you could make this a fortification?

Fortification would be a solution - it'd all work out very neatly there. But that would have a huge extra overhead of adding another subtype and ability to the set. I don't think that overhead is worth it.

The twos and threes you mention were precisely the point, and I still find them very appealing, but multiple people were finding this card very fiddly last night, to the extent that it was clearly one of the two cards with the biggest problems out of all those that got playtested. Refactoring needs to be ruthless.

One option is to move this to set 2, and introduce Fortifications in that set. In the meantime I could just create a different painless mana stone for this set.

Right, this was far too complicated. For the moment I've made a drastic simplification, which does eliminate most of the nice synergies, and take the power level down (it used to be [Torque] for 2 mana, now it's for 1 mana).

Text was:

­: When target land is tapped for mana this turn, its controller adds one mana of any color to his or her mana pool (in addition to the mana the land produces).
[Torque]: Untap target land.