Interestingly, the speed of propagation of acoustic waves through almost any knappable (amorphous or microcrystalline) material consistently produces that 60 degree conical shockwave pattern (of which you see only a part when the stone is hit on an edge).
Any kid who has ever shot thick glass with a low-powered BB gun has seen the complete 60-degree shock cone...
FWIW, my practice-knapping obsidian substitutes include Carerra Glass and the thick glass from the front of old CRT TV sets.
For practice on tougher, microcrystalline cherts, I substitute "johnstone" (slabs of old toilet tanks)... '-)
Interesting. I have never tried “ceramics” for knapping. Have to try that. Most obsidian around here is in round nodules. Picking the place to start is always interesting.
Thsnks for the insight!