Your questions are quite thought provoking and as such, got me to thinking
Before I tackle your questions, know that the Lore entries on many of the magical disciplines in Mizahar have been purposely left a little vague as a way to encourage players to get creative and experiment with the magic their character's use. Tarot created reimancy so of course he has the best insight into what would be possible with reimancy as a whole. That said, my answers to follow are based mainly on my own interpretation of reimancy and aren't meant to be right or wrong or binding in any official way. It's just how I see it.
Is there a "Default" Earth that is created when transmuting to earth?I don't see an actual default Earth being created or transmuted to. It kind of depends on the nature of the reimancer. If someone is very attuned to the earth, nature, forests, and such, I can picture them primarily dealing with simple dirt. Those tied to the desert would likely deal with sand. Different types of soft or hard stone may be created or transmuted if the character is more of a hardy, mountain man type.
Can reimancry differentiate from the different kinds of stones out there? For instance Marble or Schist versus Granite or Microline.As with the previous question, I would think this would depend on the reimancer's personality and what they are like as a person in general.
If one can, how does one learn new types of earth? Does one use the geology skill/auristics or can one take the instinctual approach and just be familiar or around a particular subset of stone to be able to manifest it. (The Kalean mountain ranges would have different species of stone than the stone under Cyphrus plains and perhaps a reimancer might be more attuned to either depending on where they were born or learned the skill.)What said at the end of this question actually fits well with my answers to your first two questions. Where the reimancer is born and raised would likely influence the type of stone he would deal with. As for learning a new type of earth, I could see this becoming more involved if you so wished. Remember that reimancy is all about one's personal djed. That djed is going to materialize in so many different ways depending on the reimancer. If a reimancer is feeling particular lonely, isolated or even sad, they may focus more on hard stone over soft dirt. If they are particularly angry perhaps more volcanic types of rock.
Can reimancy differentiate between soil/sand/clay and stones (Igneous, Sedimentary and Metamorphic)? If it can, would transmuted soil be mainly mineral sediment or could it be closer to what Hummus is for fertile soil?Not as much science as an art really. I see the reimancer being able to create or transmute earth that fits them in a particular way under different circumstances for different purposes. I don't imagine any real differentiating between the different types beyond altering to fit the reimancer.
While transmuting pure or alloyed metals seems like it would be out of the question, what about transmuting orestone? Such as Hematite (iron) or Galena (Silver and Lead). Similarly, what about gems, would it be possible for an earth reimancer to make these kinds of stones too?This one is a little tricky. I can see various types of ore being possible but you are right, pure or alloyed metals would not be possible. Well, maybe they could be possible but it would involve creating it as a para-element of earth and fire and it would be extremely complicated. So overgiving becomes a certainty with such efforts causing medium overgiving at best with severe overgiving being a strong possibility. As for gems, I see them as being similar to the pure or alloyed metals. Earth reimancy could create raw gemstones in small quantities though they wouldn't be worth much by themselves. Precious stones such as diamonds would be much more difficult and would also induce overgiving due to the complexity.
Would it be harder to make a diamond or easier to make the softer quartz or turquoise?Diamond would definitely be more difficult with severe overgiving being almost assured. Lesser gemstones would be easier though overgiving is still a threat if great care isn't taken. Of course, the higher your skill in reimancy, the easier it is to do more difficult things with less fear of overgiving.
Is it possible to transmute/attract fire in different colors and intensities? For instance a potassium burn for a purple color or magnesium for an intense, white light. I suppose what it comes down to is, can one's res be primed to be react to attracted fire this way and likewise be primed to transmute into these colors?The transmuting of different colored fire is more cosmetic so I could see the color being related to the nature of the reimancer. As for different intensities, I see it as the more res you "burn" the greater the intensity you can transmute or attract. A very dangerous thing though as one could easily fall to overgiving by trying to work with extremely hot fires.
Is this element strictly water? Or can other liquids be created like ammonia?I say strictly water. At much higher levels of reimancy, perhaps salt-water as a para-element but otherwise I'm going with water is water.
Can the composition of the transmuted air be altered? For instance spreading one's res throughout a small room and converting it into mainly CO2, or breath, to suffocate. Or conversely convert a confined space's used up air into fresh, breathable, standard air mix.I would think you could create air in a confined space that was previously void of it or attract/remove it from a room. I don't think you could change the composition of the air drastically without going to immediate overgiving (again, very high level reimancers would have a better time).
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Overall, I would say that more specified transmutations or attractions of specific elements or even para-elements is going to be difficult. The greater one's skill level, the easier it will get but overgiving is going to be a definite danger even for the Masters. I say this because the greater your skill, the greater the effects your going to shoot for; the more complex and complicated your desired results will be. Thus, the more powerful you become, the more dangerous overgiving is.