Pointed Discussion

Use a signet instead.

If you want to run a mono-colored deck with four-drops instead of three-drops, the diamonds aren't that horrible.

I don't think that's a diamond at all. I have used my godly planeswalker powers to harness the arcane energies of a grill briquette.

These were around prior to the signets, and as Begleone alluded to, a viable mana rock for moncolored decks. Especially EDH where we cannot use off clored things, like signets.