How often have you heard the term "Flint and Steel"? Quite often I'm sure, however there are numerous rocks harder than a 7 on the Mohs scale that will throw sparks from a steel. I went out today and found 3 types in just a few minutes..
Bottom left is Jasper, a nice opaque red colour often used in Jewelry, bottom right is good old flint and you all know how many colours it comes in, and top one is Chert in a lovely tan orange colour. All these throw excellent sparks and also add a nice bit of colour to your tinder kit!
Don't be led to believe that only flint works for percussion fire making!