Parad (Mercury ) Nandi
Pandit NM Shrimali explains, Nandi Bull in seated posture idol is made with Parad should be placed adjacent to the Shivling. According to the ancient Vedas, Parad is the most pious and auspicious metal for the worship of Gods. During the Samudra Manthan, when poison evolved out of the ocean, everyone ran away except Lord Shiva and Bull Nandi. Lord Shiva, followed by Bull Nandi came to help. Lord Shiva was the only one who can resist the deadly poison. Lord Shiva drank the whole of the poison. As he drank the poison, the descent of the poison was stopped by his divine consort. Therefore Lord Shiva is also called as Neel Kantha. Some poison fell out of the mouth of Lord Shiva, and Nandi drank it off the ground. The Devas and Asuras who also were watching the scene were struck by wonder as what would happen to Bull Nandi. Then Lord Shiva said them that Nandi Bull had devoted itself so much to the service of him that he had all the powers and protection of Lord Shiva.
Shrimali ji says, Nandi signifies strength, faith, devotion and constancy of belief. Images of Nandi usually are outside the Shrine of Lord Shiva, facing Lord Shiva. Often the image of the Nandi Bull, the vehicle of Lord Shiva, faces towards the Shivling. Nandi represents single-minded devotion and its peaceful strength. Nandi is considered as the guard and protector of all the Shiva Temples. Dharma or virtue is associated with the cow and added maleness, Nandi is an official guard of Lord Shiva. Pandit Ji says, Worship of Parad Nandi is sure to transport true prayers of the devotees to Lord Shiva. It provides the devotee with a life full of respect, devotion, loveliness, fame, pleasure and intellect.