For more than a century, evidence for the production of crucible steel in Central and Southern Asia, prior to the European Industrial Revolution, has fascinated and challenged material scientists, historians and archaeologists. At the same time, chromium-alloyed stainless steel was developed in the early 20th century, building upon 19th century experiments with low chromium steel. Here we demonstrate new evidence of the intentional addition of chromium to steel nearly a millennium earlier, as part of the Persian crucible steel (pulad) tradition including the production of low-chromium crucible steel in early 2nd millennium CE Persia. We analysed archaeological finds from the 11th c. CE site of Chahak in Iran showing the intentional and regular addition of chromium mineral to the crucible charge, resulting in steel containing around 1 wt% chromium. A contemporaneous crucible steel flint striker held in the Tanavoli Collection is reported to also contain chromium, suggesting its origin from Chahak. We argue that the mysterious compound ‘rusakhtaj’ from Biruni's (10th – 11th c. CE) recipe for crucible steel making refers to the mineral chromite. Additional historical sources up to the mid-2nd millennium CE refer to crucible steel from Chahak as being particularly brittle, consistent with its increased phosphorus content.