I never used the cut Agate-Flints myself. Do they work? Sure sounds like it. One I did see was in a lock over in Taylor's shop. It appeared to be sharp, but did not spark well - in that lock. He replaced it with a Tom Fuller napped flint and the lock came alive.
I saw some for sale at the local store, seems to me, they were about $5.00 each. each one, visible in the blister pack, had what appeared to be fault lines. I bought some caps for the 14 bore and carried on. I use black English Flints sometimes, I have a few French amber flints that seem to work in both my Verner and Tenn. rifle, but I use Rich's river rocks (chert, it seems) - they are wonderful, love em, while they last. Still have a bunch, thankfully.