The Collaborative International Dictionary of English v.0.48:

Rupert's drop \Ru"pert's drop`\
   A kind of glass drop with a long tail, made by dropping
   melted glass into water. It is remarkable for bursting into
   fragments when the surface is scratched or the tail broken;
   -- so called from Prince Rupert, nephew of Charles I., by
   whom they were first brought to England. Called also
   Rupert's ball, and glass tear.
   [1913 Webster]