Form of steel with a high tensile strength that rusts to a grey-brown, almost purple, crust far stronger than the original metal in about 18 months, going through many interesting and beautiful color changes in the meantime. Originally intended to be used in bridge building, it's become a much-used metal for sculpture, high-end domestic architecture, and even boats. Its only drawback is that the rusting tends to make long stains on concrete and the finished surface is matt and textured, thus rendering it unusable for such things as say, tracks on a railroad. Objects made of it can last for an estimated 20,000 years.
Rumored to be the inspiration for Rearden Metal, of Atlas Shrugged fame, and some other s-f "miracle metals", it was originally produced only in North Haven, CT. I used to pass by the yard where all the sculptures were curing in the weather...it was a beautiful and eerie sight in the middle of the industrial wasteland.