This is referred to as "the mark of God".
Can anyone figure out why? Or what it is used for?
Here is a little history:
The finds of Mohenjo Daro and Harappa have revolutionized the idea of the movement of civilization. Once the idea was cherished that civilization moved from West to East with the migration of the Mediterranean race in that direction. Now it has been established that civilization moved from East to West. The worship of Shivalinga was the characteristic feature of the Indians. The ancient Egyptians, Greeks and Romans borrowed Linga worship from the ancient civilized Indians...
From the discoveries made by the Indian Archaeological Department, it is clear that from 200 BC onwards the veneration of the Buddha and Jain stupas was very common and popular. On the slender evidence of this is suggested that the Shaivas, many of whom were converts from Buddhism and Jainism, might have imported it into Shaivism. The shape of the Linga also is advanced as apoint in favour of this argument. But the finds of Mohenjo Daro and Harappa gave a line direct to this complacent idea by proving the existence of Linga as early as 3000 B.C. If this is so, we are to reject this interpretation as fanciful, for it evinces that the Linga-worship was in existence in the second century B.C.