There exists a glass structure that can withstand the blows of a hammer without even cracking! The drops go by two different names; 'Dutch Tears' and 'Prince Rupert's Drops.' They are said to have originated in Holland, hence the 'Dutch Tears' name, and they were studied extensively at the Royal Society of London, where they got their English name.
The drops are made rather easily. The process involves dropping molten glass into water, which causes the outer glass to harden quickly, and also allows for the long, thin tail to form on the end of the drop, giving it a tadpole appearance.
The science behind the glasses incredible strength is not as simple. Essentially, the inner glass remains hot after the outside cools, and the slow cooling process causes constriction which increases the glass's hardness! This hardness allows for the bulbous end of the drops to be struck with a hammer without cracking!
One of the odd things about the structures is that they have an "Achilles' heel." If even the smallest part of the drop's tail is damaged the entire thing will crack and lose its strength! It seems counter-intuitive, but any damage allows for all the potential energy to leave the structure, leading to breakage.
