sedimentary
Sandstone with Manganese Oxide Patina
Clastic Sedimentary Rock (primarily SiO2)
Hardness: 6-7 (Mohs scale), Color: tan/buff interior with black exterior coating, Luster: dull/earthy, Crystal structure: granular clastic texture, Cleavage: none, Specific gravity: 2.2-2.8
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Physical properties
Hardness: 6-7 (Mohs scale), Color: tan/buff interior with black exterior coating, Luster: dull/earthy, Crystal structure: granular clastic texture, Cleavage: none, Specific gravity: 2.2-2.8
Formation & geological history
Formed from the accumulation and cementation of sand-sized mineral grains or rock fragments. The dark exterior is a 'desert varnish' or manganese/iron oxide coating formed by slow mineral precipitation over long periods in arid or semi-arid environments.
Uses & applications
Common building material, aggregate in construction, and occasionally used in landscaping. This specific weathered specimen is mainly of interest for geological study or amateur collecting.
Geological facts
The dark coating, often called desert varnish, can take thousands of years to form and is often composed of clay minerals combined with iron and manganese oxides concentrated by microbial activity.
Field identification & locations
Identify in the field by the grainy, sandpaper-like feel of the interior and the thin, dark weathering rind on the surface. Commonly found in riverbeds, deserts, and mountain outcrops. Collectors look for unique weathering patterns and shapes.
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