Fulcrum

In the stone age flint was a much sought after resource. In the UK there were Grimes Graves where neolithic miners mined in chalk for flint. This flint would be traded all across the UK and possibly into europe. Even after the discovery of metal, miners would still work here for flint being cheaper and easier to acquire than the metal. They would use deer antler for mining. I'm proposing/suggesting the following. For gravel to no longer drop flint and just occasionally drop rocks. Then for the implementation of bone/antler tools, these can only dig sedimentary rock slowly and metamorphic even slower, not being able to touch igneous rock. The slow speed of metamorphic digging means one can only reasonably use it to harvest individual pieces and not go seeking ores or dig too deep. Then for flint nodules to spawn as blocks in veins inside different rock types (chert and chalk ), these would have the hardness of metamorphic rock but drop several pieces of flint on breaking. Flint could then be knapped into something harder and faster than igneous tools but less durable than metal tools. One could also make it more likely to cause collapse to avoid people being underground bases with it, limiting them to pits or quarries. This would provide players who wish to stay in the stone age more options for tools and would provide early metal players with a more expanded tools set and allow them a means of gathering stone/cobble for primitive building or foundations. Bone tools could also be used as a means of gathering early copper similar to or be an alternative to the gold panning.