Quartzite

Hardness: 7 (Mohs scale), Color: tan to light brown, Luster: vitreous to dull, Crystal structure: granoblastic (non-foliated), Cleavage: none, Specific gravity: 2.6-2.7

Formed from the metamorphism of quartz-rich sandstone. High heat and pressure Recrystallize the quartz grains into a dense, interlocking matrix. This specimen appears water-worn, likely from a fluvial (river) or coastal environment.

Used as crushed stone in construction, railway ballast, and occasionally for decorative stone or cladding due to its extreme durability and resistance to weathering.

Quartzite is so hard that it can scratch steel and glass. It is often more durable than the surrounding rocks, which is why it frequently outcrops on ridges or remains as well-rounded river cobbles like this one.

Identify in the field by its hardness (will not be scratched by a knife) and its grainy, sugary texture under a hand lens. This specimen was found near Sao Paulo, Brazil, where Precambrian metamorphic basement rocks are common.