The story clearly and repeatedly pronounces the word as "chronosteen", which is how the iPlayer subtitles spell it as well.
Talk about it here.
Chronosteel was a metal that could only be forged within the Time Vortex. Because of this, even a small piece could change the course of history. Three pieces of Chronosteel at different points in the history of Earth had the potential to cause the entire planet to be sucked into the Time Vortex.
The Shopkeeper forced Sarah Jane Smith, Clyde Langer and Rani Chandra to different points in time to recover them. Sarah Jane was transported to 1889 where the Chronosteel took the form of a key to a door. Clyde was taken to 1941 where the Chronosteel was believed to be Thor's Hammer by Adolf Hitler and was used a power source for the Nazis' Sieg Transmitter. Rani was taken to 1553 wherein the Chronosteel took the shape of a knife. Once the pieces were brought together in the same time period, the danger passed. (TV: Lost in Time)