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The hardness of a rock is related to its diagenetic history and mineral content. These rocks are sedimentary rocks - limestone, sandstone, etc. If one has a source of metamorphic or igneous rocks from which to fashion tools then carving these figures is simplified. Even a tightly cemented sedimentary rock can be used to grind or carve and flint is a sedimentary rock that has multiple documented uses as a tool. Needles, awls, spear and arrow heads, axes, hammers, etc.

If you have the time to do something like this and the imagination then I'm sure you could produce these carvings using things you found on the ground pretty easily.

You would need a rock that is harder than the rock you carve into. Possibly a „Hand axe“ made from flint would suffice:

https://en.m.wikipedia.org/wiki/Hand_axe

Flint chips very easily though. I always assumed it was more for cutting meat. I guess I could see a really thick "hand axe" being used for very primitive wood carving.

Harder stone?