The son of the wind-god,Hanuman, apprised Sri Rama(avatar of god Vishnu) about the discovery of Sita(avatar of goddess Lakshmi),on returning to Kishkindha (from Lanka).Then Hanuman delivered to Sri Rama the jewel for the head(Chudamani), earlier handed-over by Sita as a token:-
Then, untying the bright divine jewel for her head tied in her garment, Seetha gave it to Hanuma, saying that it might be handed over to Rama….5-38-69
Hanuman also conveyed Sita’s message to Sri Rama. Sita’s words:-
This beautiful jewel for the head has been well-guarded by me. You also ought to recall a mark indeed painted by you (by slip) on the curve of my cheek with powder of an orange-red rock, when my former mark on my forehead got effaced." 5-65-22
Sita, further said:-
This beautiful jewel, which has its origin in sea-water, has been sent to you. Seeing this in my grief, I am feeling always happy as though I am seeing you."-shloka 5-65-23.
Sri Rama, on seeing that excellent jewel,spoke to Sugreeva as follows:-
This excellent jewel was presented by my father-in-law to Seetha when she had become a bride and was tied to her head in such a way that it looked additionally beautifying.—shloka 5-66-4
Sri Rama, further described the jewel, as under:-
The jewel, which was found in the waters and recommended by the pious, had been presented to him(king Janaka) earlier by the intelligent Indra the lord of celestials, who was highly pleased in Yajna, a sacrificial rite (intended to propitiate him).--shloka 5-66-5
From the foregoing, it can be inferred that the jewel for the head or the Crest-jewel(Chudamani) worn by Sita was earlier obtained during the Samudra Manthan(churning of the Ocean of Milk) and kept with demigod Indra. And Indra, at a later date, gave it to king Janaka on completion of a yajna(sacrificial rite) by king Janaka to propitiate him.
Which specific yajna(sacrifice ritual) was conducted by king Janaka to propitiate demigod Indra?
What are the details of the story of Indra giving the Crest-jewel(Chudamani) to king Janaka?
Reference- Valmiki Ramayana,Book Five-Sundara Kanda