Carbon nanotubes used in Medieval sabres

Scientists say that by empirically optimizing their blade-treatment procedures, 17th century craftsman in Damascus managed to create nanotubes which gave Damascus steel blades almost magical characteristics:

To Europeans, Damascus steel blades seemed magical. Not only could they cut a piece of silk in half as it fell to the floor, they could cleave rocks and their own swords without losing sharpness. The problem facing sword smiths was how to produce steel that was both hard and malleable. Too much carbon and the steel is hard and brittle; too little and it is too soft and malleable to hold an edge when sharpened. Damascus steel blades were forged out of small pure cakes of steel containing around 1.6–1.7 per cent carbon, called wootz. Produced in India, wootz cakes were shipped to Damascus where expert sword smiths fashioned them into blades.

The skill had been lost by the 18th century, when ore and impurities supplies ran out.