sedimentary

Conglomerate (polymictic, possibly metamorphosed)

Polymictic conglomerate; clasts cemented in a dark, likely quartz-rich matrix

AI-generated identification · may be incorrect

Rounded to subrounded pale and dark clasts in a black matrix, with contrasting quartz/feldspar-rich fragments; typically hardness 6–7 for quartz-bearing components, dull to weakly vitreous luster, no consistent cleavage, and variable specific gravity around 2.5–2.8.

Identified September 12, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed when water-rounded gravel and cobbles accumulated in a high-energy stream, alluvial fan, or shoreline and were buried and cemented, commonly by silica or iron-bearing minerals. The exact geological age cannot be determined from the photograph; the texture could also reflect low-grade metamorphism if the clasts are slightly flattened.

Uses & applications

Used locally as dimension stone, decorative aggregate, landscaping rock, and geologic teaching material; attractive polished pieces may be collected, but it is not ordinarily a gemstone.

Geological facts

The defining feature is its rounded clasts, distinguishing conglomerate from breccia, whose fragments are angular. The unusually dark matrix may contain chlorite, biotite, magnetite, or other mafic minerals.

Field identification & locations

Look for rounded, pebble-sized fragments that differ compositionally from the matrix and cannot be scratched easily with a steel knife if quartz-rich. Common settings include ancient river deposits and foreland basins; a hand lens and fresh broken surface help distinguish it from concrete.