Blue-black saligram increase peace and harmony in the family where yellowish saligram increase wealth and prosperity. It is aid there are so many symbols of saligram equal to Lord Vishnu's several names. Different symbols and chakra formation is not by nature, it is made by the river worms called Vajra-Keeta. It's teeth/nails are said to be harder than granite stone. The chakra formation is found not only on the outer surface of the Saligram. There are chakras inside Saligram also. This can be seen through the hole or holes on the Saligram with the aid of torchlight. The holes are termed as vadana (mouth). Chakras can be seen inside these holes. The Vajra keeta worms drills the stone, get inside and live there. During their stay inside the tones, they cause these chakras. Similarly outside the worm also causes Chakras only. Saligram are used as living place by the worm not uses as shells.
Sri Saligrams is considered the direct symbol of Lord Vishnu, They are found only in Mukti chhetra and Damodar Kunda (north-west of Nepal). According to the religious text of Devi Bhagwate (and other scriptures) to kill demon Jalandhar Lord Vishnu have to destroy Sati Brindha's sati dharma. When he did that Sati Brindha gave four seeming desecrations to Lord Vishnu to become stone, grass, tree, plant. To wash away this reaction Lord Vishnu took four avatars (incarnations). He became stone (Sri Saligram) grass (Kush) tree (Pipal) and plant (Tulsi). Since this time the Saligrams are considered to be most auspicious to behold and to worship. The worshipper knows no fear and by Sri Saligram's mercy the worshipper is blessed to attain all desirable things; worldly comforts, good wife, good sons, good health and wealth etc. It is all by the blessings of Lord Mahavishnu that His pastimes are being served. It is an excellent service for devotees of Lord Narayana.