In the Padma Purana Lord Siva declares the greatness of Saligrama Shila as follows:
Mallinga Kotibhi Drsthi Yad Phalam Pujiti
The merit obtained by seeing and worshipping hundreds of thousands of My Lingams, is equivalent to that obtained by worshipping one single Salagram shila (stones).
If this is what Lord Shiva says, lets know where the Saligrama are found, what they are, their legend, types, Pooja and benefits etc., from the links given on this side
Most of these are either natural products or hand made products and hence there will be so minor difference in size shape colour etc. and they cannot match the photographs 100%