Maṇisūkāra Jātaka (No.285)
The Bodhisatta lived in the Himavā as an ascetic, and near his hut was a crystal cave in which lived thirty boars. A lion used to range near the cave in which his shadow was reflected. This so terrified the boars that one day, they fetched mud from a neighbouring pool with which they rubbed the crystal; but because of the boars’ bristles, the more they rubbed, the brighter grew the crystal. In despair they consulted the Bodhisatta, who told them that a crystal could not be sullied.
The story was told in reference to an unsuccessful attempt by the heretics to accuse the Buddha of having seduced Sundarī (q.v.) and then brought about her death. J.ii.415‑8.