sedimentary

Omarolluk (Omar)

Siliceous greywacke concretion

AI-generated identification · may be incorrect

Hardness: 6-7, Color: Dark grey to brown, Luster: Dull, Crystal structure: Microcrystalline quartz, Cleavage: None, Specific gravity: 2.6-2.7. Characterized by rounded hemispherical pits on the surface.

Identified August 31, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed as carbonate concretions within siliceous greywacke during the Proterozoic eon in the Belcher Islands of Hudson Bay, Canada. The softer carbonate dissolved away over time, leaving the characteristic pits.

Uses & applications

Primarily valued by rock collectors and as an indicator erratic used by geologists to trace glacial movement.

Geological facts

Omarolluks are glacial erratics that were transported by the Laurentide Ice Sheet during the Pleistocene epoch. Because their source rock is confined to the Belcher Islands, they are excellent markers for tracking the flow of glacial ice across North America.

Field identification & locations

Easily identified by the distinctive dark grey, rounded shape with one or more perfectly circular, hemispherical pits. Often found in glacial till, riverbeds, and beaches across central North America, far from their origin.