The stones which appear in that place are called Shalagram shila. According to the Hindu culture, this stone is the shelter for a small insect known as "Vajra-Keeta" that has a diamond tooth that cuts through the Shaligram stone and stays inside it. The Saligram stones are 100’s of million years old when the Himalaya was an ocean floor. The marks on the Shaligram gives it a unique significance, the Discus of Lord Vishnu.
They are generally jet black/blue in color and do not have golden and crystal impressions on them. Some small-sized Shaligrams with white impressions found at the Gandaki river since they are small and can roll down along with bigger jet black Shaligrams. All the Shaligrams which are jet black are categorized in the "Auspicious Shaligram category" on www.shaligram.org. The donation of these shilas is less comparatively. Devotees, please note that now even black Shaligrams are not available in Gandaki river since the villagers around the Gandaki river fish them out and sell them in the market.
Benefits of Auspicious Saligrams: