What is unbreakable glass made of, and is it really unbreakable?

The presence of micro cracks on the glass surface (called Griffith’s micro cracks) makes glass breakable under tension or impact. Such micro cracks are unavoidable as the glass surface comes under tension while cooling from a high temperature into various shapes. However, one can increase its impact resistance by various methods, so it can withstand even the impact of a bullet. The idea is to remove the surface micro cracks by chemical etching, or putting the glass surface under compression by physical or chemical toughening. One can even laminate two such toughened glass plates with resin in-between. These glasses are unbreakable, but when the threshold value is very high, it eventually breaks or cracks.