Prince Rupert's drops have the shape of raindrops. These days they are made by heating the end of a soft glass rod with a Bunsen burner. When the glass melts, it is allowed to drop into a bucket of cold water from a height of about 4.5 feet. Usually the glass drop shatters when it hits the water. But occasionally the glass hardens, forming a Prince Rupert's drop. Surface tension makes the glass very hard. The head of the drop can withstand being hit with a hammer; a really strong blow will just dent the hammer. But the tail of the drop shatters easily, causing the entire drop to shatter.

So far (about 350 years), the drops have been just a novelty. Each drop has a unique shape. There has been no way to protect the sensitive tail from shattering the entire drop. Digitizing each drop and 3D printing a customized holder that protects the tail, could enable exploitation of the extreme hardness of the head of the drop.

http://en.wikipedia.org/wiki/Prince_Rupert%27s_Drop