While wandering along the valley you should keep an eye out for the curious black spiral stones known as Saliagrama that can occasional be found scattered amongst the far less interesting stones on the bed of Gandaki River.
According to many Hindus, these curious stones are representative of Lord Vishnu in his first earthly incarnation. Vishnu came to Earth in the form of a fish known as Matsya, representing the formation of protoplasm and invertebrates. In this avatar he swam into the hands of king Manu, who was washing in the river, and warned him of the great flood that was coming to wipe out all life. Manu built a boat and took on board the "seeds of life" which Matsya then towed to the safety of a mountaintop.
The stones, found only in this area, are very holy to believers and are not treated like icons but are in fact worshipped as deities in their own rite, as they are self-manifest and not man-made. Because of this, the rules for keeping them are very strict. They must be worshipped daily, never placed on the floor, and the keeper must wash, adhere to a strict Vaishnvaite diet, and abstain from bad practice before using the stone in ritual.
Others claim that the stones are the lingam symbol of Shiva, the goddess Durga in her Binde Basini Bhawati avatar, or most controversially that they are the 180 year old fossilised remains of Jurassic era molluscs known as amenities. According to this, somewhat far-fetched, theory this area was at the bottom of the Tethup Sea until 55 million years ago when the Indian continental plate collided with the Asian continental plate and the Himalayan mountain range was born.
Although you will see them for sale on many souvenir stalls, you should not purchase one, not only because of the complex etiquette involved in keeping a deity in your home but for the more practical fact that the government has banned their export in order to preserve the fossil record.