Heatlessly melting them into a unified product doesn't
strike me as a trivial thing to do, even assuming magic
Why not? Neither heat nor magic are involved in the formation of sedimentary rock.
A ton of time and rather specific conditions are, though.
I'm not asking the elementals to create anything. Simply to rearrange existing material. Nobody would have a problem with it if we asked an air elemental to give us wind to move our ships. The elemental is causing the material of its element to move. We don't actually imagine a human-shaped body of air standing up against the sail and pushing.
To rearrange it on a very, very fundamental level. Humans can't turn a pile of calcium into bones just because they're made of bones, and it's not a simple transformation regardless just because they're chemically the same.
If you wanted an earth elemental to move some earth around, or move some things that happened to be in the earth, I don't think anyone would have more of a problem with it than a wind elemental generating wind for your sails. I think this is more akin to asking a wind elemental to produce isolated hydrogen for you, or perhaps precipitate liquid water out of the air.
But it probably can generate infinite fire. Fuel is not fire. Asking a fire elemental to generate fuel doesn't make sense. But if you give it fuel and ask it to make more fire...it should have no problem with that. I'm giving earth elementals soil and earth, and asking them to use that to make stone...via exactly the same processes that the earth does. Just faster. I think that's reasonable.
This does sound reasonable, but I still don't think it'd be as quick or simple as a fire elemental lighting things on fire or a wind elemental blowing on something. A fire elemental turning wood into charcoal might be a better analogy- doable, but definitely a full-on crafting action.
Of course, this does assume these elementals are of the fairly magical variety. Some depictions of them just have them as blobs of element with no particular abilities otherwise.
Ok. So...whether it's the "spirit of the earth" that's forming the body of an elemental, or us as wizards forming that elemental body...either way, the shaping of earth is a necessary part of that process. I don't really care if we use elementals, I just want to make rocks.
Making rock that sits there and does nothing should be much easier than making rocks that stand upright and walk around.
Right?
Well for starters, I'm not sure earth elementals are made of solid stone; I usually envision them being extremely densely packed soil.
Assuming they are stone, I'm also not sure if the material for the body is made of local materials. Starting a big enough fire to summon a fire elemental might make sense, for instance, but it might not be necessary, depending on the exact process.
Finally, if earth elementals are made of stone, and this stone does come from what used to be dirt on the spot you summoned it, you've still got the potential for the human-steak problem; just because it can form its own body out of it doesn't mean it can utilize the same process on command.
If it's the mages who are doing the shaping, and earth elementals are made of stone, and the stone comes from local dirt, then yes, I would say your mages can probably made dirt into stone with at least as little effort as it takes to summon said elementals. That still wouldn't necessarily make it a trivial or even feasible process, though- you presumably cannot make an entire building out of earth elementals in a day (at least, I would assume not- I actually have no idea), which leaves the rate of elemental-less stone production with a rather large possible range.