Shiva linga though many people in their ignorance consider it as a phallus worship or union of male-female etc. these ideas are absurd. A linga means leenam artham gamayathi, that which points out to the hidden truth, and shiva means auspiciousness. A shiva linga is a pointer to the Truth. Truth is the most auspicious. In many place the linga is in the form of a ellipse. But there are temples where the ling is in the form of a circle, square, rectangle, pentagon, hexagon or an octagon. And there is a temple in south tamilnadu (neat pudukkottai / arandangi), Avudayarkoil, the swami is called Atmanatheswara who has no form, Arupa. This is a temple built by Sri Manikkavacakar. Leave this alone, one of the jyothirlinga, Kedar has the linga in the form of a small mountain.
The shiva linga which is baneswara is the one which is brought from the river narmada. This is a naturally formed linga because of the rolling of the stone, in a small water fall. And the other lingas are either made from a stone from narmada of any good stone, like marble, crystal etc. Otherwise there is no “upadhi”, attribute difference between a baneswara and any other iswara. But, there is an important difference, for a baneswara there is no prana-prathishta, establishing of vital-air (deity) through sacred chants required and for the other including a linga made from the narmada stone.
There are different types of lingas that are worshipped. The results are attributed to be different for each. Linga made of crystal, is a special form for the Brahmins. Others include narmadeswar linga, linga made from a metal gold, silver etc. There is a special linga made of mercury amalgamation which is considered to be good. There is one called hariharatma linga which is one found in a shaligrama stone. A shaligrama stone is like a bana linga which is found in the gantaki river flowing in Nepal. Inside this, when a shaligrama is bigger, there is a shivalinga form stone which is considered very sacred. We can make linga from clay or cow dung also.
It is also to be understood here, that when someone calls a shivalinga a Swayambhu, it does not mean, swayam bhavati iti, that which came automatically / naturally, except in the mythological stories. But, that for which the one who installed it in the first place is not known.