Boulders are breakable objects, which stun enemies in one hit if either thrown or used as a melee weapon, and break upon impact.
When Raziel holds a boulder or an urn, he cannot glide, swim or pickup weapons. He can however walk underwater. This enables the Glyph menu so he can cast any spell, which is not normally possible when swimming. It is unknown whether this was intended or is a possible bug.
The boulders in the early beta version were unbreakable and could be used repeatedly.