I was reading this article on Top Ten Lost Technologies and came across an interesting tidbit about Damascus Steel, one of only two materials to make the list (the other being Roman Cement). Damascus Steel was a super-strong Middle Eastern forged metal used from roughly 1100 A.D. to 1700 A.D. It was said to be able to cut through rocks and other people's swords, making it the bad-ass material of its day.
Sadly, the "recipe" for making Damascus Steel no longer exists, and they've not been able to reverse-engineer how to make it. In any case, below is the passage that struck me, boldface mine:
The particular process for forging Damascus steel appears to have disappeared sometime around 1750 AD. The exact cause for the loss of the technique is unknown, but there are several theories. The most popular is that the supply of ores needed for the special recipe for Damascus steel started running low, and sword makers were forced to develop other techniques. Another is that the whole recipe for Damascus steel--specifically the presence of carbon nanotubes--was only discovered by accident, and that sword smiths didn't actually know the technique by heart. Instead, they would simply forge the swords en masse, and test them to determine which met the standards of Damascus steel. Whatever the technique, Damascus steel is one technology that modern experimenters have been unable to fully reproduce.