Just going to leave my reply here because quote is messed up:
Ooh, that sounds like a good idea! So, for instance, you can transmute rock or metal into a weapon, but you couldn't transmute something like water into a weapon because they don't fall under the same element!
Yes. That's the idea.
really well.
Now I want to write a story surrounding that idea. :P
Yes, they are interesting ideas. Though I was thinking: Would it be better if we didn't stick with the basic four, but made a more extended list: Water (liquids), air (gases), fire (which would represent energy in its non-material form I guess), Earth, which could represent only solid nonmetals, while metals had their distinct 'element', (and one should not forget about plasma). So, what I basically propose is that maybe metals and nonmetals should be distinct. What do you think of that?
Also, Mercury? What would we do with that? Is it liquid, metal or is it a convenient material that's both and can be used for both water and metal/earth alchemies?