It is glass, but not normal one, it is a strange phenomenon that happens when melting glass at very high temperaturen got put in cold water and solidified very fast in this drop shapes, becoming harder then steel. If you shoot it with a gun, the projectile would be deflected, but this is only true on that round side. If you slightly move the tail all the drop will immediately explode. There are video explaining the phenomenom better than me 😅🤙

The common term used to describe it is called a Prince Rupert's drop.

It's makes the glass practically impossible to break except for the small focused area at the end (some people call it the tail).