Narmada Shiva Lingams come from the Narmada River in Madhya Pradesh, India, one of the seven sacred holy places of pilgrimage in India. The collection of these stones is passed down from generation to generation. The Shiva of Narmadeshwara Lingam stones are sacred in both Hindu and Buddhist belief. They are gathered on one sacred day of the year from an area near the source of the Narmada about 300 miles north of Mumbai. They are polished into the traditional oval/egg shape by the action of the river.

According to legend, a dip in the Ganga River is supposed to cleanse one’s soul, seven dips in the Yamuna is supposed to do the same, but mere sight of the Narmada has the same effect. The Ganga herself is believed to visit this river once a year, in the guise of a black cow, to cleanse herself of all her collected sins.

The Narmada River, also called the Rewa, from its leaping motion (from the root rev) through its rocky bed has been mentioned by Ptolemy and the author of the Periplus. The Ramayana, the Mahabharata and the Puranas refer to it frequently. The Rewa Khand of the Vayu Purana and the Rewa Khand of the Skanda Purana are entirely devoted to the story of the birth and the importance of the Narmada River which is said to have sprung from the body of Lord Shiva. It was created in the form of a lovely damsel who enamoured the gods and was hence named by the Lord “Narmada” – delight giving. It is also often called Shankari, daughter of Lord Shankar (Shiva).

There is a popular Hindi saying “Narmada ke kanker utte Shankar” which means “the pebbles of the Narmada turn into a manifest form of Shiva” refering to the fact that the pebbles in the bottom of the river take the lingam shape.

Narmada lingas are also called Banalingas. There is a story narrated in the Aparajita-pariprchchha (205, 1-26) about the origin of the bana-lingas or Narmada lingas and their association with the Narmada river. Siva wanted to destroy the ‘tri-pura’, which had been obtained as a boon by the arrogant demon Banasura, and he let go a fiery dart from his great bow ‘pinaka’. The dart broke the three ‘puras’ into tiny bits, which fell on three spots: one on the hills in Sri-kshetra (of unknown identity), another on the peaks of Amarakantaka in the Vindhya ranges, and a third on the banks of the holy river Narmada. The bits that fell in these places soon multiplied into thousands, each bit becoming a linga. As they formed part of the possession of Banasura, they were called Bana-Lingas. Amarakantaka, the peak in Madhya Pradesh, is in close proximity to the source of the river Narmada, which according to the puranas, originated in the Vindhya mountains and flowed in the Kalinga country.

The Shivalings found in the Narmada are made of crypto crystalline quartz, and are said to have been formed from a meteorite that fell to Earth several million years ago. They are also called Svayambhu Lingas, meaning in Sanskrit “Self-existent mark or sign of God”, because they are discovered in nature and not carved or crafted by human hands.What Shaligrams are to Vishnu, the Bana-lingas or Narmada lingas are to Shiva: sacred and self-manifest representations.

Banalinga is a part of the fivefold family of deities (Panchayatana). The five Hindu deities Shiva, Vishnu, Devi, Surya and Ganesha are the embodiment of 5 bhutas/tatwas worshipped in formless stones, which are obtained from 5 different rivers . Panchayatana form of worship is said to have been introduced by Adi Shankara, the 8th century C.E Hindu philosopher

In this tradition five stones each representing the respective deities are placed on the sacred altar for the Panchayatana worship. Sun God Surya is represented by a crystal found in Vallam in Tamil Nadu. Mother Goddess Shakti is represented by the Swarnamukhi stone found in Swarnamukhi River in Andhra Pradesh. Vishnu is represented through Salagramas found only in the Ghantaki River in the Himalayas. Ganesha is represented by the red Shonabhadra stone found in the river bed of the Sone River flowing into the Ganges. Shiva is represented by the Bana Lingas found in the Narmada river bed near the island of Mandhata.

The Narmada Shivalingas represent the blending of male and female energy, with the shape of the stone representing the male (knowledge), and the markings on it representing the female (wisdom). This blending of Shiva and his consort or energy, Shakti, is essential for the growth of the human race. The lingas also illustrate harmony through duality. In these shivlings is the energy of divine acceptance of what is, as it is. They represent power, creation and the physical plane and help to ground the energy of their worshippers.

The fiery energy of Shiva’s penance, or tapas, called jwaalaagni continues to blaze all over and from him. By bathing the lingam, symbol of Shiva, with cooling substances like milk, water, yogurt and cooling, fragrant herbs we keep him cool and balance out his fiery energy. This practice of bathing of the diety is called Abhisheka in Sanskrit and Lord Shiva is known as Abhisheka Priya, or one who loves Abhisheka, so the more we bathe him, the happier he is.

http://www.rudraksha-ratna.com/narmada-shiva-lingams_464.html5
http://en.wikipedia.org/wiki/Banalinga
http://www.occulttreasures.com/bana_linga.html