In Game Description
The rare gem called tearstone has the
uncanny ability to sense imminent death.
This blue tearstone from Catarina boosts
the defence of its wearer when in danger.
Boosts defense by 50% when health is under 20%
It cannot be used to survive attacks that would've otherwise killed you, as the effect does not apply to the damage taken that trigger the ring's buff. The defensive buff only occurs after the ring has already been activated.