Since noone has really made an attemp on the stats of said rock body/earth elemental/stone golem, I'll give it a try. Usually one can provoke a lot of good comments/critique by making a rough sketch
Blood to Rock, Bone to Stone
Muto Corpus (Terram) 40
[tab][/tab]R: Touch, D: Sun, T: Individual
[tab][/tab]You turn your body into solid stone, and although you’re overall proportions remain unchanged you are not readily recognizable and your new form no longer appears human. This transformation increases your weight roughly 5 times; resulting in an increase of +2 Size, without altering your overall dimensions. This increases Strength by +4 and the wound category ranges by two steps as usual. Rather than suffering the standard -2 penalty to Quickness however, this shape suffers double that since the increase in Size is entirely due to bulk. Thus the stone form suffers a total of -4 Quickness slowing both movement and reaction speed. On the other hand, the hard solid mass makes it very hard to damage, grating an increased +15 Soak (ignore Soak from armor in this shape). This Soak may be reduced by 5 for blunt weapons though; such as Hammers, Maces and also Picks. Finally your stone arms can double as Gauntlets using the Brawl ability in combat when unarmed. Note that this newly gained weight makes it very difficult to climb and espcially swim - and should you find a horse willing to carry you; it might not be able to. Also you would take care not to cause stairs and floors, as well as boats and bridges to give way underneath the density of your body. When casting the spell you clench a small stone in your fist. The stone held determines the type of rock you turn into, and allows you to end the spell by dropping it to the ground.
[tab][/tab]Base 25, +2 Sun, +1 moving stone body
A short description on how I got here. The design is inspired by various sources including my own attempts to stat out the stone bear in the previously discussed 'stone bear spell', but also the Bjornaer Inner Heartbeast stone elemental power, some assumptions based on Size rules in the core rulebook as well as the existing MutoCorpus spells.
Changing the weight and Size seems given. Personally I'm a fan of increasing Strength along with Size (contrary to the Inner Mystery for example) as with the normal rules. I cannot see the stone shape move around and support itself without increased strength.
Also I have reduced Quickness by more than the standard (slightly inspired by the Inner Mystery), since the stone man didn't feel slow enough otherwise. The argument here is that in the same way that one Size increase potentialy raises Strength by +2; so is Quickness potentially lowered by -2 for the added bulk. Usually a Size increase also affect the targets porportions though including; larger reach for better Initiative - longer limbs for improved movement speed etc. Thus, the standard Quickness penalty is reduced to -1 per Size step up. In this case the Size increaese is solely due to added weight, thus the -2 per Size step. I deliberately designed this to be able to cater for future needs, in case someone in my troupe decides to do something similiar. Or given time maybe my Magus' Study Requirement will force him to spend an entire year in the form of a Golem of Gold in order to get some real Terrram study done, increasing his wieght roughly tenfold - and what happens then? Well, +4 Size, +8 Strength, -8 Quickness. Done.
In terms of offensive capabilities, I decided against going for damage based on the magnitude of the spell (as some suggested), and although i like the notion of arms with the wieght of warhammers (where did you find this in Rop:F again MarioJPC?) - I just don't think its justified. Warhammers a often harder than stone, their shape is different and their heads larger than a normal fist. A normal hammer, or even a mace would fit the discription better IMO. But still you need the shaft in order to build up the momentum of a weapon, something you cannot hope to achieve with your own arm. I like the idea that for Brawling this shape wouldn't need armaments, so I added the part about the gauntlet, which also seems correct enough to be allowed.
Soak is probably the change most important to balance since it could easily be, well imbalanced. The core rulebook mentions that as a tree trunk you are nigh impervious to mundane weapons, so I'd assume the same goes for a stone body. Adding +15 Soak (as per the Inner Mystery) seemed fair to me. Combining this with the increased Size categories you would have a really hard time wearing this guy down with wound penalties, and you would need twice as much damage as normal to inflict the killing blow. Thus I decided to add to the discriptions that armor doesn't add to Soak while in this shape (obviously) and the part about blunt weapons and picks. The latter seems a bit out of Ars context (splitting damage types into categories that is), but it has been done before - see Doublet of Impenetrable Silk. So against a Warhammer this spell would provide roughly the same protection as a full chain mail suit. Metal is stronger than stone, but there are no 'joints' in the stone mass armor and its strength is not limited by its thickness. This seems rather fair doesn't it? Using GotBF for Soak one could achieve +6 Soak for at spell of this magnitude, so is +15 even too much? The big difference here is the Target: Individual/Part, affecting the entire body or only the outer layer of skin. I'm not talking about the 'lost' magnitude of GotBF though, but about the fact that there is a limit to the amount of Soak you can gain through a thin layer of hard skin - compared to a solid stone mass. On the other hand, you still appear human with GotBF, whereas this spell clearly marks you as a supernatural being.
Balanced? True? Enough flavour?