Narmada lingam or narbadeshwar shivalinga is dedicated to god mahadev(god shiva). It is found in narmada river. It is one of the creation of god and gift of nature.
There used to worship god shiva a great devotee, narmada, in bank of rewa river/narmada river. She used to worship god shiva by making shivalinga from sand and mud from narmada river. Lord shiva boon her that all stone found in narmada river is named with your name.