A rare nickel-sulfur particle that can be present in float glass and, in some fully tempered lites, can contribute to delayed spontaneous breakage.
Wolfmann and cmglee, via Wikimedia Commons · Image source · Image source
Meaning and boundaries
An unexplained break is not proof of a nickel sulfide inclusion; edge damage, surface damage, contact, loads, and installation conditions are often more plausible causes.
Why it matters in glass restoration
Technicians should avoid attributing every tempered-glass failure to nickel sulfide and should document alternate causes and uncertainty.