This is a video of Destin from Smarter Every Day detailing the unique mechanical properties of a Prince Rupert's drop, which is made by dripping a string of molten glass in a bucket of cold water (not to be confused with a Prince Albert's drop, which is made by piercing a man's wiener and inserting a piece of jewerly). Destin previously demonstrated the properties of a Prince Rupert's drop by smacking them with a hammer back in 2013. Now he's shooting them with a gun. Let me copy/paste some Wikipedia for you since I can't even explain how to use a vending machine without overpaying and getting your arm stuck in the chute.
The water rapidly cools the molten glass on the outside of the drop, while the inner portion of the drop remains significantly hotter. When the glass on the inside eventually cools, it contracts inside the already-solid outer part. This contraction sets up very large compressive stresses on the surface, while the core of the drop is in a state of tensile stress. It is a type of toughened glass.
The very high residual stress within the drop gives rise to counter-intuitive properties, such as the ability to withstand a blow from a hammer on the bulbous end without breaking, but experiencing explosive disintegration if the tail end is even slightly damaged.
While the head of the drop can be hit with a hammer or struck with a bullet without breaking, when any portion of the tail is damaged, the large amount of potential energy stored in the internal structure is released, causing fractures to propagate through the glass toward the head at very high speeds which breaks the entire structure into flakes and powder.
Pretty fascinating, right? The video of those things shattering from the tail down is amazing. They look like sperm exploding. "Oh no, somebody beat me to the egg, initialize self-destruct sequence!" Just kidding, sperm don't explode. Mine do sometimes tie themselves in knots though. "No, the egg is this way. NO, IT'S THIS WAY. Wait a minute...this is a plastic cup!" Haha -- later guys, I just made $40!
Keep going for the video, but skip to 2:00 if you just want to see the shootings.
Thanks to Charlie H, who agrees they should make smartphone glass out of just the heads of Prince Rupert's drops like that makes any sort of sense to anybody but me or him.