This card is representative of all regular manastones. A degenerate cross of the Moxen and Black Lotus, their casting cost reflects the difficulty in obtaining them and the exile cost keeps them from being broken.

"Degenerate" as in useless? I can't imagine why someone would ever play these, except perhaps with Mishra, Artificer Prodigy.

Degenerate as in strictly worse than it's inspirations. Originally had the cost as when I wrote the comment. Maybe should go back to that?

How does this look compared to the original design (the rest of the cycle is unaltered as yet for comparison)?

Yes, this looks fine now. As Lifespark Manastone are phrased, they don't do anything except wash a little mana and save it from one turn to the next. This design is now a Charcoal Diamond, which was always fairly weak but was sometimes played anyway, with a nice Ebon Stronghold twist to it.

Excellent, I really wanted to get these to a playable level as thematically, manastones are very important to the backstory. Glad I got there with your help.

Isn't there a rule that says that any land can be a functional 2-mana artifact? If thee's already a land version of this, it should be fine.

It's more of a guideline than a rule. But yes, you're right, this is precisely Ebon Stronghold as a 2-mana artifact, which thus means its power level is about right.

(For those who've not heard of the rule: compare Talisman of Progress with Adarkar Wastes, Prismatic Lens with Shimmering Grotto, etc.)

Nice catch Alex, this and the rest of the cycle are indeed artifact reprints of the Fallen Empires sac-lands. I am a big fan of the old sets - they being from when I was a full-time magic player (went into semi-retirement after the original Mirrodin block) and I have fond memories of 'working around' some of the old 'less-good' cards.