Thinking about mana dorks in Hearthstone. There exist a bunch of minions that ramp you a mana crystal, and even of that only ramps you while it's alive, but I don't there's any that work like MTG dorks (i.e. needing to activate)

This is almost the opposite of a mana elf, mind you. A more direct implementation would be... give you a crystal but cost you if it attacks?

But then you can just use it before you attack. Unless you mean like "battlecry: gain a mana crystal until this minion attacks," so it has to never attack? Given the limited board space, that feels so much worse than just a Rampant Growth variant

Well, this is "Attack, and I get a crystal"
Where the mtg equivalent is "Do not attack, I get a crystal"

And "Use it before you attack" was kinda there as a deliberate nod to trying to do clever things with it, but staying on-curve before combat is hardly a clever thing.

Hearthstone also doesn't have an explicit combat phase. Each minion can attack whenever you want during your turn

Mmm. Which makes it even worse. I don't think hearthstone has a keyword for it; but it has secrets which trigger on "When a minion attacks" so it could probably use that wording?

What the hell, apparently this card came out for real the same month I made this design. Escaped Manasaber

Congratulation. :)