The phallic emblem of Śiva made of precious gems was considered the best in the Kṛta age; of pure gold in the Dvāpara; of mercury in the Tretā and of earth in the Kali age.
Just as the celestial river Gaṅgā is the oldest and the most excellent of all rivers, so also is the earthen phallic image of Śiva the most excellent of all.