Strengthen stone can reinforce any stone construction (house, tower, wall segment, aqueduct, etc.), against physical damage. The DM adds +4 to the structure's saving throw against any type of damage, from siege engines to natural earthquakes. The stone object gains a saving throw against the earthquake spell (see the PHB). The spell can be cast only once on any stone object.
If the priest casts this spell on a stone golem or other animated stone being (like one created by animate rock), the creature receives a -1 bonus to its Armor Class and adds a +1 bonus to its saving throws for the duration of the spell. Strengthen stone has no effect on earth elemental or galeb duhr.
The material component, a diamond chip worth at least 500 gp, must be crushed and sprinkled on the construction.
Notes: Uncommon spell for druids.