More properly speaking, this was the ring the Haughty Knight of the Heath gave his damsel.
Young Percivale, just having left home for the first time to find King Arthur’s court, stole it from her in (willful?) misinterpretation of his mother’s parting instruction.
I cannot tell what eventually became of it; while the Haughty Knight and Percivale speak of it before they finally fight, they seem to say nothing more of it after the battle is decided.