I do not recall there be rules about breaking items in the core book (and I do not have it with me) however, I believe the way it would be handled would be via damage reduction and wounds.
This is due to the following, from the book of earth:
Groves of Stone
Area of Effect: Circular barrier 15’ in radius
... It entreats the Earth spirits
to draw an array of huge stones up from the ground,
erupting in an instant through most intervening physical
obstacles (such as the floor of a house) to create a circular
barrier around the caster. The stones are too strong to
break through with anything short of a siege engine
(against a siege engine they are considered to have 500
Wounds and Reduction 15)...
This suggests that objects that can be broken most likely have a DR and Wound Level that can be broken with either specialized tools, or appropriate weapons.