Most temple lingams are carved from black stone or marble to a size fixed to the sanctum they will occupy, but a particular category, banalingam, smooth ovoid stones taken from the bed of the Narmada river, are used uncarved, valued precisely because they are considered self formed rather than shaped by a sculptor's hand.
Installation and daily care
A carved or river-found lingam is installed with the same consecration rite, prana pratishtha, used for other murtis, performed by a qualified priest, after which it receives daily ritual bathing, abhisheka, with substances that can include milk, water, honey, yogurt and sacred ash, poured over it in a fixed sequence by temple priests as part of the temple's regular schedule rather than as an occasional event.
Who is permitted to touch the sanctum image
In many temples the principal lingam in the inner sanctum is touched only by initiated priests carrying out these daily rites, while ordinary devotees view it from outside the sanctum or receive prasad, offerings returned after being presented to the deity, rather than approaching the image directly. Smaller personal lingams, kept in a household shrine, carry none of this restriction, and are handled and bathed by the householder themselves as part of daily devotion, a much less formal practice than the temple rite.