Prince Rupert's Drops Are Toughened Glass Beads Created By Dripping Molten Glass Into Cold Water, Which

Prince Rupert's drops are toughened glass beads created by dripping molten glass into cold water, which causes it to solidify into a tadpole-shaped droplet with a long, thin tail. 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 a hammer or a bullet 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

Prince Rupert's drop - Wikipedia