We all know that Karna was given a Kavach and kundal by his father Lord Sun. Kavach was for his protection. But what was the role of Kundal?
Karna's kundalas (ear rings), along with his kavacha (mail/armour), make him immortal. His ear-rings arose from amrita. It is due to both that Karna cannot be killed in battle.
Vaisampayana continued, "Although thus urged with various words by Karna, still, O chief of the Bharata race, that Brahmana did not ask for any other boon. And although Karna sought to pacify him to the best of his power, and worshipped him duly, yet that best of Brahmanas did not ask for any other boon. And when that foremost of Brahmanas did not ask for any other boon, Radha's son again spake unto him with a smile, 'My mail, O regenerate one, hath been born with my body, and this pair of ear-rings hath arisen from Amrita. It is for these that I am unslayable in the worlds.