What is nickel sulphide inclusion?

Nickel sulphide inclusion is the unintentional incorporation of nickel sulphide, an impurity, into the glass-making process. According to the Glass Association of America, 50 different types of inclusions have been identified. Almost all of them are harmless. Nickel sulphide, however, can cause the glass to spontaneously shatter, weeks, months or even years after installation.