I have a 7 inch tile saw from Harbor Freight that I use. I have cut chert with it, but the saw has a hard time cutting through it. It seems to be a little under powered. That being said, I have tried to cut larger pieces of chert (around 4 inches thick). The largest cut I can make with this saw is around 2 1/2 inches, so I have to cut one pass, flip the rock over and try to line up with the first cut while cutting from the other side. With smaller diameter material, like creek gravel, it may work just fine since it does not to cut through all the mass.
It may be possible to use it to cut smaller pieces off creek gravel to shape them or make them more knappable. I am off work tomorrow, and if I get a chance, I will do some trim work on some Missouri creek gravel and see how it does. I have some Mozarkite gravel, and I its about as hard of a chert as I know of. If it will cut that gravel, it will cut most any type of chert.
As far as cutting the softer materials (obsidian and glass) the saw chews through those like its nothing. I have slabbed a lot of obsidian and slag glass with it.
If i get a chance to cut some creek gravel I will let you know.
Neal