Mason's Eye fires every so many crits. On Kindled Keystone Relic it reads: Every 5th critical strike cracks the target's guard, building Crumble for 60% of the hit.

Word for word it is Every {count}th critical strike cracks the target's guard, building Crumble for {value}% of the hit, and the number in it grows with the gear: 60 to start, 4 more for every item level, and it only goes off every 5 times you set it up.

It leaves Crumble behind, which makes it Stone build material.

3 bases carry it.