Quartzite Cobble

Hardness: 7 (Mohs scale), Color: reddish-purple to maroon due to hematite inclusions, Luster: dull to waxy (sub-vitreous), Structure: granoblastic, non-foliated.

Formed through the regional metamorphism of quartz-rich sandstone. The location in North Dakota (~48.2°N, 101.3°W) is part of the Williston Basin, where these smooth cobbles are commonly transported glacial erratics from Precambrian shields.

Used in construction as road ballast, as decorative garden stone, and in some industrial glass manufacturing when pure.

Quartzite is so hard and durable that it often survives multiple cycles of erosion and transport, which is why it remains as smooth, rounded cobbles in glacial till and riverbeds long after softer rocks have disintegrated.

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, rounded appearance. In the Minot, ND area, these are ubiquitous in glacial gravel deposits.