Mithila King Janaka’s wife Sunaina was the mother of Sita and her sister Urmila married to Lakshman. After King Janaka found baby Sita under the earth while ploughing the field one day and adopted her as his first born, he was blessed with his one and only biological child, Urmila. Legend has it that Sunaina was the daughter of a serpent, a devotee of Lord Shiva. And when she married King Janak, Shiva gifted her two precious two jewels, Chudamani and the other encrusted on a ring which Sunaina passed it on to daughter Sita when she married Lord Ram.
In another narrative it is mentioned that when King Janak met King Dasharath for the wedding of their children as per tradition, he had to present a gift to his samdhi and asked his wife to bring the best jewel from the treasure box for the father of the groom and Sunaina brought the finger ring presented to her by Lord Shiva. The forwarding story is that when Sita comes to Ayodhya after marriage her mother-in-law Kausalya has to give her muh dekhi and requested her husband Dasharath to gift their most precious jewel and Dasharath took off the ring presented to him by King Janak and handed it to his daughter-in-law.
Chudamani and the finger ring remained with Sita as blessings from Lord Shiva till as long as she lived. Sita wore them during her exile and when she left Ayodhya again because they were jewels of divine origin and not from the royal treasury.