On the left is my main Salagram Sila, whom I lovingly call Gopal. On the right is Ugra Nrsimha. An Ugra Nrsimha Salagram has these characteristics: They are somewhat flat, They have a large whole for a mouth, inside the mouth are two "chakra" facing each other, one on the top and one on the bottom.
Salagrams can be identified as different forms of the Lord according to Their natural shape and markings. The markings are all natural, as found at the Kali-Gandaki River.