While ferrocerium-and-steels function in a similar way to actual flint-and-steel in fire starting, ferrocerium actually takes on the role that steel played in traditional methods: When small shavings of it are removed quickly enough, the heat generated by friction is enough to ignite those shavings. The sparks generated are in fact tiny pieces of burning metal. In traditional flint-and-steel fire-starting systems, using actual flint, tiny shavings of the steel that are removed in the striking process, rather than the actual flint, are what burn. The origin of its easy sparking is cerium's low temperature pyrophoricity, its ignition temperature occurring between 150 and 180 degrees celsius.
Since smaller scrapings become better sparks, the mechanical properties of rare earth metals must be adjusted to give a usable material; to that end, at least two strategies have been developed to make such alloys more brittle:
There were actually three different Auermetalls developed: The first was just Iron and Cerium, the second also included Lanthanum to produce brighter sparks, and the third added other heavy metals.
A modern ferrocerium firesteel product is composed mostly of iron, combined with an alloy of rare earth metals called mischmetal (containing approximately 50% cerium, 45% lanthanum, and small amounts of neodymium and praseodymium), plus a small amount of magnesium: