The overwhelming majority of Shiva worship is directed not at an image of a god with a face and limbs but at the linga: a smooth vertical shaft, usually stone, set into a circular or squared base called the yoni. It stands in the sanctum of every Shaiva temple. Milk, water, honey and bilva leaves are poured over it continuously, and at the great sites the stone has been worn visibly concave by centuries of that pouring.
The word linga means mark or sign. The classical theological reading is that it is precisely a sign and not a portrait: Shiva is held to be beyond form, and the linga is the minimum shape that can stand for a presence without describing it. The Lingodbhava story makes this explicit. Brahma and Vishnu argue over which of them is supreme, and a column of fire appears with no visible top or bottom; Brahma flies up as a goose and Vishnu digs down as a boar, and neither reaches an end. The point of the story is the failure.
The anatomical reading, that the linga is a phallus and the yoni a vulva, is not an invention of outsiders: the Sanskrit words carry those senses, some texts develop them, and the association of Shiva with generative power is old and real. But Shaiva devotees very largely do not experience the object that way, and a good deal of colonial writing treated the anatomical sense as the whole truth and the theological sense as an embarrassed later cover story. Both readings are present in the tradition, they are weighted quite differently by different schools, and the atlas does not adjudicate between them.
Twelve jyotirlinga, lingas of light, are the principal Shaiva pilgrimage sites, from Somnath in Gujarat to Rameshwaram at the southern tip. At Amarnath in Kashmir the object of pilgrimage is not carved at all: it is a stalagmite of ice that forms and melts each year.