I'm stuck for an ore again, only this time it's Quartzite. I've got most of my low-voltage machinery set up, with several battery buffers, but I'd like to monitor the amount of energy I've got in those batteries easily. To that end I installed Nuclear Control 2, which has a bunch of information panels. Unfortunately, the recipe for the Gregtech sensor kit involves quartzite, and I haven't seen any.
This feels like it's going to be a recurring thing. At least with the tetrahedrite I could be reasonably sure it would be visible from the surface. For veins where the generation is lower, that's not an option.
I've seen lots of coal, magnetite, bauxite, salt / lepidolite veins (handy for lithium), redstone / ruby veins, and a couple of monazite veins, but there are lots of things I've never seen.
I've found it in Green Swampland and in Meadows, usually just below the surface in cave/caverns around 52ish in the open.