A while ago we were talking about kidney stones. One of the variety of things that kidney stones can be comprised of is Struvite. To the chemically inclined, it is ammonium magnesium phosphate.
But today I want to mention one of struvite's other incarnations.
Struvite crystals will grow in tinned fish, especially salmon. Looking for all the world like pieces of windscreen glass after a crash, the crystals will come in for testing as "glass found in a tin of fish".
But it is not glass. And it is not harmful.
Should you swallow some it will dissolve in your gastric juices.