a question regarding suitable materials for knapping

[BillA]

humor me guys, I don't even know how to phrase my questions

recently saw a hacha or celt/axe, ~8" OAL with damage to the blade and its subsequent repair by flaking



pecking is clear on all sides, but the blade and tip are ground



my questions relate to the material used
the axe has been laying on the surface of the jungle for 1000 years or so, and all stone does weather
where old moss was attached seems slightly higher suggesting some protection ?
but the rock to me appears to be consolidated clay, super fine grain w/o inclusions
good for pecking and grinding but not flaking
but the repair, keeping the CL of the blade constant, was done by flaking

what defines if a stone can be flaked ?

thanks

edit: closest I could find was https://geology.com/rocks/novaculite.shtml but seems much harder

 

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[ToddsPoint]

I just now saw this old thread. The answer is that the rock has to be able to be chipped with conchoidal fractures. Flint, chert, and obsidian all chip with conchoidal fractures. Now here's the surprise. Igneous rocks also chip with conchoidal fracture. Difficult but possible. All of these rocks have one thing in common. They have no definite grain, like wood does. Their strength is the same in every direction which allows a knapper to determine the direction the flake is to come off. Your celt looks like a fine grained igneous rock to me, pecked and polished. Gary