Just to be very clear, with modern ferrocerium / mischmetal (whatever) "firesteel" the composition of the striking edge doesn't matter. A broken piece of glass works perfectly well.

From my discussion of the matter at http://www.equipped.org/devices28.htm

The artificial flint used for such purposes is similar to the flints used in traditional cigarette lighters, but it is a somewhat harder alloy in order to give off hotter and more long lived sparks. It is comprised of a mixture of metals and rare earth elements, by weight approximately 20% Iron (Fe) with trace amounts, less than 3% each, of Zinc (Zn) and Magnesium (Mg) and the remainder a combination of rare earth elements, 50% of which is Cerium (Ce), the remainder primarily Lanthanum (LLa) and Neodymium (Nd) and trace amounts of some other rare earth elements. These are alloyed at high temperature and then extruded into rods of various diameters. When scraped with a hard, sharp edge a thin layer is scraped off and the resulting friction heats the scrapings up to the point of ignition, giving off an impressive shower of very hot sparks. Note that this scraper doesn't have to be steel, but the edge does have to be hard and sharp enough to scrape with. A broken piece of glass can be very effective, for example. The back edge of many knives works as well as the sharp edge, if it hasn't been eased.

By comparison, natural flint is a very hard quartz mineral, harder than most steels, which when struck on a sharp edge by steel or iron creates small sparks by removing and heating up the softer metal. These sparks are relatively weak and few in number, so making a fire with these requires a fair amount of skill and special tinders. Natural flint is a real pain in the you-know-what to use compared to the man-made variety. Fine for those re-enacting the experiences of the Old West's Mountain Men and the like, but not very practical for us today. Some manner of man-made flint should be part of every survival kit.