Quartzite

Hardness of 7 on the Mohs scale, typically brown, tan, or white in color, vitreous to greasy luster, non-foliated crystal structure with interlocking quartz grains, conchoidal fracture, and specific gravity around 2.65.

Formed through the metamorphism of quartz-rich sandstone under high temperatures and pressures, commonly occurring during tectonic mountain-building events. Given the location near Michigan (44.6°N, 86.0°W), this is very consistent with glacial drift deposits originating from the Canadian Shield or local Paleozoic formations.

Used in construction, road aggregate, manufacturing of silica brick, and sometimes as a decorative stone or tumbled into polished stones.

Quartzite is extremely durable and resistant to chemical weathering, which is why it often survives long-distance glacial transport and tumbling in rivers and beaches to form smooth pebbles.

Can be identified in the field by its extreme hardness (scratches glass easily), inability to be scratched by a knife, and the way it breaks across quartz grains rather than around them. Common in gravel bars, beaches, and glacial till throughout the Great Lakes region.