The drops of Prince Rupert are produced by dropping melted glass drops into cold water. Water quickly cools and solidifies the glass from the outside. This thermal quenching can be explained using a simplified model of a fast chilled sphere.
Umm, there are interesting details that have led to research over the centuries. When the tail is cut, the drop bursts into the powder, while the bulbous head can withstand up to 15,000 newtons (3,400 lbf).
These droplets are characterized internally by very high residual stresses, which give rise to counter-intuitive properties, such as the ability to withstand a blow from (e.g. a hammer) on the bulbous end without breaking, while exhibiting explosive disintegration if the tail end is even slightly damaged. In nature, similar structures are produced under certain conditions in volcanic lava and are known as Pele’s tears.
Prince Rupert didn’t discover the drops, he played a role in their history by bringing them to Britain in 1660. He gave them to King Charles II, who in turn delivered them in 1661 to the Royal Society (which had been created the previous year) for scientific study.
In a Prince Rupert’s drop, the interface between the inner and outer regions deflects the forces sideways, so the crack can’t propagate. Also, if the tail is broken, the shallow cracks in the tail shoot parallel to the axis of the drop, deep into the head, and into the interface. The damage is so great and causes an explode.
1994 - Credit: Purdue University
“The tensile stress is what usually causes materials to fracture analogous to tearing a sheet of paper in half,” says Purdue postdoctoral associate Koushik Viswanathan. “But if you could change the tensile stress to a compressive stress, then it becomes difficult for cracks to grow, and this is what happens in the head portion of the Prince Rupert’s drops.”
Recently, researchers at the University of Bristol and the University of Iceland have studied the glass particles produced by explosive fragmentation of Prince Rupert’s drops in the laboratory to better understand magma fragmentation and ash formation driven by stored thermal stresses in active volcanoes.
Also, I found an interesting video: Crushing Prince Rupert’s drop with hydraulic press