"A rain of dirt, gravel, and small pebbles fills the area, dealing 2d6 points of bludgeoning damage to every creature in the area. This damage only occurs once, when the spell is cast. For the remaining duration of the spell, this debris covers the ground, making the entire area difficult terrain. At the end of the duration, the rocks disappear, leaving no aftereffects (other than the damage dealt)."
If stone call could affect objects, the spell text would support that. Also, the saving throw line would read Saving Throw: none(object); instead of just "Saving Throw: none"
To be clear: Stone call does not damage objects.
I'd have it damage unnattended objects, if they're particularly fragile.
I agree. The spell lists Spell Resistance as 'NO' so the stones aren't a magical effect like Magic Missile. Despite being created by magic they're real rocks. Apply the damage against item hardness.
I doubt they'd do any damage against a ships sails. As it says on the SRD...
Certain weapons just can't effectively deal damage to certain objects. For example, a bludgeoning weapon cannot be used to damage a rope.