>>12821473Yup, it does look like quartzite. This is what happens to sandstone (=sand that has consolidated into a solid rock) after it gets exposed to huge pressures and temperatures (like when two continental plates crash with each other). The original sand grains disappear and the quartz (the mineral they were made out of) crystallizes again as tightly packed microscopic grains, forming the rock you see now.
It's a very common rock so I guess some kid picked it from the ground and threw it onto your neighbour's roof.
