In 1422, King Charles VI died after ruling France for more than 40 years. Also remembered as Charles the Mad, the king may have been the first person to exhibit the glass delusion; that is, he thought he was made of glass and would break. The delusion, brought on by melancholia, would continue to pop up through medieval Europe until the late 19th century.
Hippocrates took it upon himself to define melancholia in his tome "Aphorisms" as a long-lasting "fright or despondency." Think of it as profound depression. The condition affected those who were isolated and not generally around a lot of people. People with the melancholia-induced-glass delusion also often experienced photophobia, or an extreme sensitivity to light.
The French King Charles VI was so convinced of his glass fragility that he insisted on having special clothes made for him, reinforced with iron on the inside so that he wouldn't shatter with the movement required of his station. He also wouldn't let people touch him, for fear of breaking, something that persisted through his life.
Charles VI wasn't the only royalty with the glass delusion. Princess Alexandra Amelie of Bavaria was convinced that she'd swallowed a glass grand piano as a child that had reconstituted her somehow into fragile glass. She, too, took care with her movements to avoid "breaking." (This is the case Stuff You Missed in History Class discuss in their podcast.)
According to a 1990 History of Psychiatry paper reviewing this type of melancholy, others afflicted by such delusions thought they were flasks (now called urinals), oil lamps or some other glass receptacle.
The combination of photosensitivity and fragility factoring into delusions at this time in history is thought to have been partially due to the invention of clear glass. People had never seen clear glass before, and they viewed it as something almost magical.
In that 1990 paper, author Gill Speak writes that melancholiacs whose delusions manifested in a need to protect the body often were preoccupied with protecting the soul, too. The glass delusion may well have been a side effect of dealing with the metaphysical reality of death and self-preservation.
These issues haven't disappeared in today's world either. If you're isolated from people and suffering from a mental illness, it sort of makes sense to imagine yourself as a material that's easily breakable and largely invisible.
Today, though, delusions often reflect more recent technological or cultural advances. In fact, a 2011 study published in the International Journal of Social Psychiatry found that, since 1950, the most common delusion came in the form of persecution complexes and the belief of being spied on. Since the advent of the Cold War and the evolution of today's technological landscape, it's no wonder that delusion took hold. And if Edward Snowden is even half right, maybe a fear of being spied on isn't that delusional after all.