Weathered iron-rich quartzite (probable)

Hardness about 7 Mohs; pink, reddish-brown, and gray; generally dull to subtly glassy; massive, granular texture with no obvious cleavage. The reddish colors likely reflect hematite/goethite, while gray areas are recrystallized quartz.

Formed when quartz-rich sandstone was metamorphosed under heat and pressure, welding quartz grains into a hard rock; later weathering produced the iron staining. Precise age cannot be determined visually; regional Brazilian quartzites may range from Precambrian to younger metamorphic units.

Used as crushed aggregate, road stone, building and paving stone, and occasionally as a lapidary or decorative specimen. This piece has limited jewelry value unless it displays attractive polish or patterning.

Quartzite typically scratches ordinary glass and resists acid. Its rusty-red coloration is secondary iron-oxide alteration rather than a separate red mineral.

Identify it by its very hard, sugary quartz texture and fractures that cut across grains rather than around them; sandstone usually crumbles along grain boundaries. Around 22.7°S, 43.5°W near Rio de Janeiro, weathered Precambrian metamorphic rocks and quartz-rich rocks are plausible, though a field locality or hardness test would improve confidence.