Spell of Humility
The Spell of Humilty is a spell cast by the Roman Magus in C.E. 10 causing gargoyles' clothes and other items on their person to enter stone sleep with them, thus preventing nudity upon awakening at dusk. The spell was clearly quite powerful since it managed to affect every gargoyle in the world (including those the Roman Magus was presumably unaware of such as the Mayan Clan) and continues to affect them throughout all time beyond C.E. 10. The Roman Magus used a combination of various magical artifacts to achieve this amazing feat.
It seems the spell has specific limits however. If a gargoyle is holding a living creature, that creature will not turn to stone with them. Even certain items will not turn to stone if (from the gargoyle's point of view) the item is not their property. This explains why Hudson's sword sometimes does and sometimes does not turn to stone (Hudson comandeered the sword from a Viking in 994).
Tidbits
Behind the scenes, the spell was a convenient plot device used by Greg Weisman to explain why an otherwise natural process would have such an unusual and unnatural side effect. Weisman has joked that the Roman Magus "must have had the backing of ALL of Rome's Standard's and Practice's Executives". [1]
Rumour has it that Brooklyn may be in Caesar Augustus' court as the spell is cast on his Timedancer journey.