Unbreakable glass

Unbreakable glass is glass, or glass substitute, which does not display the normal fragility of glass – in general the term is not used to refer to something that is absolutely unbreakable.

In history
According to Petronius ( c.  27 AD – c. 66 AD) in his work Satyricon, an inventor  brought a drinking bowl to the Roman emperor Tiberius made of vitrum flexile - translated as either flexible or unbreakable glass - which did not shatter but merely dented. Tiberius asked if anyone else was aware of the invention. When the inventor replied that he was the only person who knew the secret, Tiberius had him killed. The motivation, according to Reynolds, was to protect the livelihood of the glassmakers.