I realize now that perhaps my comment was a bit misleading. The term "spark" needs a bit of clarification as it can refer to two totally different phenomena. There's the electrical type of spark and the falling ember type of spark. When a crystal is broken it produces the electrical type of spark which will flash and cause the crystal to "glow" briefly. When high iron is struck it frees small particles and exposes them to oxygen and those particles auto-ignite causing the ember type of sparks. Iron is a pyrophoric, which is a substance that will catch fire when it is exposed to oxygen. White phosphorous and lithium are in this category as well. The thing that makes iron different is that a chunk of iron won't auto ignite(unlike the other examples). Without getting too involved the short answer of why this happens is that a piece of iron is coated with a layer of iron oxide(rust) which protects the raw iron underneath from the oxygen. When iron is struck with flint or pressed against a spinning grindstone small pieces of pure iron are torn free and exposed to the O2. They immediately oxidize(turn to rust) Fe + O2= FeO2 + heat. The pieces are so small that all the heat generated in that reaction can't be absorbed by the unexposed iron in their center or the surrounding air. Thus the glowing embers. It also explains why some embers burn longer than others. The larger sparks have more surface exposed to the air as well as a little more pure iron in their centers which hold the heat a bit longer. If the particle of iron gets too big it will be able to absorb the heat generated by it's own surface oxidation and won't get hot enough to glow/burn.
If your are getting falling embers, there is iron present. If you are getting a flashing right at the point of impact you are breaking crystals.
I hope my explanation was clear enough to make sense. In a super simple nutshell, iron sparks are heat based while crystal fracture sparks are more electrical/ light based.
While flint is in fact a crystal(microcrystaline quartz, many tiny crystals) it's the iron in the steel that makes the heat. The reason you need the flint is that quartz is harder than the steel. Using a flint and steel produces both kind of sparks, but the tiny electrical sparks from crystal fracture are hard to notice with all the ember type sparks that also result. Steel is used instead of pure iron because iron is too malleable to produce free falling particle when struck.
Geology, chemistry, and physics all before lunch. I'm ready for recess.
