I was wondering do you modify some spells based on the surrounding materials? IMO almost all of the time it should be adjusted up wards and not down to help the PC's. But it can just as easilly be adjusted down if they are trying to create a wall of stone out of sand.
For Example: A Mage is casting wall of stone while standing on a granite ledge. IMO it should have more HP and be stronger then the base spell description. So look in GM Law and use the material mod for granite.
MDC
