In the same year [21 AD], a certain artist discovered how to temper glass, making it flexible and ductile. And when he had been brought before Tiberius Caesar, he offered him a small bottle; at which the emperor was indignant, and threw it down on floor. But the artist picked the bottle up from the floor, which had become bent like a vessel of brass, and taking a hammer out of his bosom, he beat the bottle into shape again. When he had done this, Caesar said to the artist, ” Is any one else acquainted with this manufacture of glass?” And when he had affirmed with an oath that no one else understood it, he ordered him to be decapitated, lest, if the art became known, gold might come to be esteemed as clay, and all the value for the precious metals entirely destroyed ; because, in fact, if glass vessels could not be broken, they would do better than either golden or silver ones.
-The flowers of History, especially such as relate to the affairs of Britain by Matthew Paris
And indeed, there are a few recollections of this story in history. The so-called flexible glass (or vitrum flexile) has never been found but there are several theories as to what it was. Perhaps borax had been discovered in Rome and used accidentally in a batch of glass. But 1500 years before it was used in the middle-ages by goldsmiths? Some people have even suggested the radical idea that it was an early form of plastic!
The more popular (and feasible) theory, however, is that the glass was in fact just extra strong and the legend grew from exaggerations.