What's a mana rock?
There are quite a few artifacts in Magic, going all the way back to the original set, that generate mana. Since we can only play one land per turn under normal circumstances, cards that let us "get around" that rule speed us up (ramp). Driving up the ramp lets us play bigger spells than we should be able to, earlier in the game. As for why they are called "rocks," most of the cards in question are thematically stones, rock, jewels, or something similar.
To run this analysis, we have to make a few assumptions. With 40 lands in your Commander deck and using your mulligan to get to 3 lands in your opening hand, you will almost always make your first 4 land drops. Someone who is better at math than I am can figure out the exact probability.
Here's a simple example:
Mind Stone costs 2 mana to cast and taps for 1 mana. So, by turn 2, you've played 2 lands and then tap out to play the Mind Stone. Technically, you can tap the Mind Stone that turn and use the 1 mana for something else, but this analysis is more concerned with what happens on the next turn.
Turn 1: Land
Turn 2: Land, Mind Stone
Turn 3: Land
With 3 lands in play and the Mind Stone, you can make a total of 4 mana on the 3rd turn.
Here's another example:
Turn 1: Land
Turn 2: Land
Turn 3: Land
Turn 4: Land
Turn 5: Land, Gilded Lotus
Turn 6: Land
With 6 lands in play and the Gilded Lotus, you can make a total of 9 mana on the 6th turn. (The likelihood of making all 6 of your opening land drops is low. But with some other mana rocks on earlier turns, ramping into the Gilded Lotus is more reasonable.)
To recap:
- Mind Stone: 4 mana on the 3rd turn
- Gilded Lotus: 9 mana on the 6th turn
This isn't meant to be a complete list of all ramp in the game. Take a look at how this works and then apply it to your own deck. You can also compare different ramp cards against each other this way.
Onto the ramp!
For raw speed, these are the most "broken" of the Commander-legal mana rocks. They all give you more mana than you started with on the turn they come into play. This is sometimes called "fast mana." Most have drawbacks, except for Sol Ring. :)
- Sol Ring: 4 mana on the 2nd turn (2 mana on the 1st turn)
- Mana Crypt: 4 mana on the 2nd turn (3 mana on the 1st turn)
- Mana Vault: 5 mana on the 2nd turn (3 mana on the 1st turn)
- Grim Monolith: 5 mana on the 3rd turn (3 mana on the 2nd turn)
In the slightly less broken category, we get these rocks. They give you less mana than you started with on the turn they come into play, but do ramp you on the next turn. These tend to not have drawbacks. Some have an upside, like Mind Stone which can be converted into a card draw or Gilded Lotus which can make colored mana.
- Mind Stone: 4 mana on the 3rd turn
- Worn Powerstone: 6 mana on the 4th turn
- Thran Dynamo: 8 mana on the 5th turn
- Gilded Lotus: 9 mana on the 6th turn
The sweet spot for rocks that ramp you only 1 turn but have an upside is at 3 mana, tap for 1. These rocks give you a smaller boost than the categories above, but almost always give you a little something extra to sweeten the deal.
- Chromatic Lantern: 5 mana on the 4th turn
- Darksteel Ingot: 5 mana on the 4th turn
- Azorius Cluestone*: 5 mana on the 4th turn
- Azorius Keyrune*: 5 mana on the 4th turn
- Obelisk of Bant**: 5 mana on the 4th turn
*There are Cluestones and Keyrunes for all of the guild color pairs as well.
**You guessed it; there are Obelisks for each of the tri-color shard combinations.
Special mention goes to Coalition Relic. It works like the rocks in the list above, except you can tap it to charge it up the turn it comes into play. This gives you 2 colored mana on the next turn.
- Coalition Relic: 6 mana on the 4th turn
