I’v been flintknapping for 30 years and attending prim tech events just as long and this is the first time I have seen the word, “radiolarite.”
Looks like pretty stuff.
Sidebar, good article for a knapper, it may wind up posted for GGG:
https://www.manchester.ac.uk/discover/news/prehistoric-brits-used-rare-rock-crystals/
Just a guess, let the other guy Google, but possibly a rock formed from radiolaria, compressed to stone like limestone is made from many sea dwelling calcium containing mini/micro fauna.