What is the shiny, glass-like substance sometimes seen in canned tuna fish?

Struvites in canned tuna are often mistaken as glass. Struvite is a phenomenon that occurs during the processing of the fish. It is made up of natural mineral components that precipitate out of the fish during the canning process. This is not a harmful situation since the struvite, if swallowed, would dissolve in the stomach. Manufacturers work hard to prevent the formation of struvite.