Nandi made beautifully in pure crystal also called as Sphatik.
Nandi is called Nandishvara is considered to be guardian of abode of Lord Shiva. Nandi (bull) is also considered to be vahana of Lord Shiva.
Always auspicious to keep along with Shivling of Lord Shiva. It is always believed that Nandi along with Lord Shiva been put helps to fulfills prayers and wishes faster.