Where to find brittle quartzite for brittle whetstone in Grounded 2

Finding Grounded 2's brittle quartzite is an essential part of progression after you've got your first optical disk all tucked away. You need the stone not just to craft a brittle whetstone for weapon upgrades, but to build the smithing station where you make those enhancements. Tracking it down is a rather more involved process than you're likely used to at this point in your survival adventure, though, as the mineral only comes from a handful of very specific locations. Initially, we thought those locations were all underground, behind root systems, but it turns out that's only partially true.