The Silver Medallion is a key item in the first game of the modern Ninja Gaiden series, found and used in "The Path To Zarkhan" chapter. Once both Gold and Silver Medallions are acquired, they are to be placed on their respective locations on an altar near previous save statue. Once placed they will cause an explosion opening a path to the next area for Ryu to proceed.
Both Gold and Silver medallions are missing in Ninja Gaiden Sigma, and are replaced with the Key Of Triton (found in sunken ship's hidden compartment) which allows Ryu access to the same area.
The Silver Medallion can be found in the area between the sunken ship area and the preceeding save statue area. In this area there is a small underwater maze, at the end of which is a treasure chest with the Silver Medallion. The Oxygen Cylinder (which allows unlimited breathing while underwater) should be acquired beforehand to ensure Ryu does not drown.
In Ninja Gaiden Sigma the Silver Medallion does not exist, however a Talisman of Rebirth can be obtained instead from the same treasure chest.