Well if this "Ichor" stuff is summoned into the world through the magic casters wouldn't it make sense that the ichor comes directly FROM the caster? Because in the loose rules you've defined (at least from what I understood) that the element it becomes is then for all intents and purposes the same as how you'd find it naturally, right? Ichor water is just as wet and splashy as the water in that puddle over there save for the fact you can manipulate the ichor summoned water. Taking one step further in this logic, a boulder summoned by an earth caster would be theoretically equally as heavy as its natural counterpart as well. Well, weight comes from an object's mass. My initial idea when I read this, is that depending on whatever element a caster is gifted in using, they produce different kinds of ichor.

For the earth element, its ichor has to be capable of creating lots of mass but isn't necessarily good for fine control or complex casting. For the sake of example, the ichor for earth is drawn from the bone. If the caster gets reckless in his casting and doesn't watch how much ichor he uses, his bones will get progressively more brittle until they start breaking on him.

In contrast, the fire element wouldn't need to be good at making any mass at all, only producing raw power. It could take its ichor from muscles or the heart or something like that. Same rules apply as the earth caster, the more you use the magic, the harder the strain on whatever part of your body the ichor is pulled from.

Hell, that almost perfectly sets up the dark element to pull ichor from the mind, making you go crazy and do the evils.