Alter Stone

Allows the caster to transmute stone from one natural type to another, such as from granite to marble.  Can transmute 1 cubic foot (in any shape) of stone for the duration.  The transmutation begins from the closest point on the targeted stone, so if concentration is broken then some amount of the destination stone type will be present.  Spell does not effect gems or minerals.  

When cast at higher levels, duration decreases by 1 minute per spell level.  Ritual casting takes 20 minutes per cubic foot.