If you are looking for a rock that will throw sparks with a carbon steel knife or a traditional steel all you need to locate is locate some quartzite cobbles. Visit a stream near your house and examine various fist or larger size cobbles. Look for one of the two following characteristics: 1) on the surface you may see scattered crescent shape discolourations on the surface, this indicates a rock that is forming a hertzian style cone from collision with other cobbles, or 2) simply crack a few rocks open using bipolar percussion and experiment until you find a rock type that will provide a reasonably "sharp" edge and try striking a spark with your steel.
Remember, that when you are attempting this method of fire starting you are literally shaving pieces of metal off your steel. This process creates tremendous heat through friction and tearing, what you see as a spark is a red hot bit of metal that has been sliced from the steel.
With this in mind, flint is not necessary, only a sharp edge. Look for rocks with a fine crystal structure. With a little experimentation you will find a local source for these rocks. Quartz type rocks are the single most abundant mineral in the earths crust, all you have to do is find the correct type of quartz. And most of them will work.
I would be using it for my 
:You_Rock_
