Strengthen Stone

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 kind of damage, from siege engines to natural earthquakes. The stone object gains a saving throw vs. the Earthquake spell. The spell may be cast only once on any stone object.

If a character 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 elementals or galeb duhr.

The material component must be crushed and sprinkled on the construction.

[Complete Druid's Handbook]

Druidic

Sphere: Elemental (earth)
(Alteration)
Level: 5

Range: 10 yards
Components: V, S, M
Duration: Permanent
Casting Time: 1 hour
Area of Effect: 1 building or wall
Saving Throw: None

Material Components: a diamond chip worth at least 500 gp (consumed)