or Prince Rupert's Drops. Glass bubbles first brought to England by Prince Rupert. Each bubble has a tail, and if the smallest part of the tail is broken off the bubble explodes. The French term is larme Batavique, because these toys were invented in Holland.
“The first production of an author ... is usually esteemed as a sort of Prince Rupert's drop, which is destroyed entirely if a person make on it but a single scratch.” —Household Words.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Rupert's Balls from Infoplease: