Changing the shape of a rock that would be low effect and the end shape need not be specified by the spell.
The end shape always needs to be specified. Spells are always precise. They will do whatever the spell description says
and nothing else. If no shape is given in the description, then no end shape exists and no change will take place.
What you can do is experiment with methods of "input". So a spell could make a rock soft as clay and allow the wizard to change the shape with his hands. Or a rock held in one held could take on the form of whatever object is held in the other hand. I'd allow that. I won't allow anything that says "whatever shape" or something like that.
Rate of change also needs to be specified for unliving creatures. If you change
anything instantly, it will break. Because if you think in terms of physics, you are applying a near infinite force.
But a rock can probably get away with a duration of 2 for a change, while a human being will probably not survive a major change with that duration. The bigger the effect and the more fragile or complex the object/creature to be changed, the more careful you should be.
