sedimentary
Greywacke (Stream Pebble with Quartz/Calcite Veins)
Greywacke sandstone / Metagraywacke
Hardness 6-7, dull to earthy luster, fine to medium-grained matrix composed of quartz, feldspar, and rock fragments, dark greenish-grey to brown color, intersected by thin resistant veins.
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Physical properties
Hardness 6-7, dull to earthy luster, fine to medium-grained matrix composed of quartz, feldspar, and rock fragments, dark greenish-grey to brown color, intersected by thin resistant veins.
Formation & geological history
Formed in deep-water marine environments (turbidite deposits) where muddy sand accumulated rapidly. Later subjected to pressure, low-grade metamorphism, and hydrothermal fluids filling fracture lines with quartz or calcite.
Uses & applications
Used locally as aggregate, road base, and decorative river rocks; primarily of interest to rockhounds and amateur collectors.
Geological facts
River and beach erosion preferentially wear down softer parts of the host matrix, leaving veins or indurated rings standing out in raised relief, often creating unusual patterns.
Field identification & locations
Look for a hard, dark grey or greenish sandstone matrix intersected by fine light-colored fracture-filling veins. Commonly found along gravelly riverbeds, glacial tills, and active coastlines.
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