When you use a flint to make sparks to start a fire, the amount of sparks and the duration of the sparks depends on the hardness of the steel you strike on the flint. That is why a Flint Fire Starter Kit is better than using a knife or a piece of steel.
Flint is a hard stone and has to be harder than the steel you use. It is the steel that makes the sparks you see burning not the flint. If you use soft steel you will create a lot of sparks but they will burn out too quickly to ignite your tinder. If you use really hard steel the sparks will last longer but you may not create a sufficient amount to ignite your tinder.
Hardness of metal is measured using a Rockwell scale. A soft metal is between 70 and 75 Rockwell C, while hardened steel is between 85 to 97 Rockwell C scale. If as piece of steel is too hard it will be brittle and break easily if it is too soft it is bendable.
If you use a knife on a flint it will leave gouges in your knife. If your knife has a serrated section on the backside, eventually the serrations will become worn reducing the effectiveness. Also you do not know the hardness of your blade, so it will be trial and error.
I have not seen any real flint for sale but manufacturers do make rods of steel with that are very effective for producing sparks. The rods are referred to as fire steel. The fire steel is sold with a small steel scraper. The hardness of the scraper is annealed to produce the most effective sparks for starting a fire.
The fire steel rods come in different sizes but work the same way. You hold the rod close to your tinder with the end pointing into the center of your pile. Start at the end away from your tinder and quickly pull the scraper towards your tinder pile applying light pressure. If you are going to use a steel fire rod to impress your friends you should practice beforehand. Experiment with different pressures and movements until you get the most effective amount of sparks.