Chert or flint is very common where I am at. It ranges in color from snow white to a dark grey almost black, and even a deep dark red. I am constantly foraging for good specimens. I use a rock hammer to break them open and test them with a fire steel. Identifying chert is not always easy when covered with dirt and the parent rock material, that’s where the hammer and steel come in.
All forms of chert that I have found will work great for a fire steel and they will work in a flintlock provided you can properly shape them; Although your lock ignition time may be slowed.
Some quartz type rocks and even granites will work for a fire steel, basically anything with a proper hardness and a sharp edge will throw a spark
A sharp edge is critical when using chert to get it to throw good sparks. One of my flintlocks has a piece of driveway chert in it right now.
Many of my fire kits that I put together have a piece of chert in them no bigger than a musket ball or rifle flint and they work just fine.
Grab a hammer and a fire steel and go have some fun finding what will work in your area, you might be amazed at what you’ve been stepping on all these years.