Struvite is the name given to hard crystals composed of magnesium ammonium phosphate that occasionally appear in canned fish, particularly salmon and tuna. They appear as small, hard, glass-like fragments in the can and are occasionally mistaken for glass contamination by alarmed consumers — but they are completely harmless and dissolve in the stomach’s acid. The crystals form when magnesium from the fish’s bones and tissues combines with ammonium compounds and phosphate naturally present in the flesh, a reaction favoured by the heat of the canning process and by storage time. Canners can reduce their formation by controlling raw material quality and processing conditions. Finding struvite is a quality complaint, not a safety incident.