sedimentary

Sandstone Concretion (Omar or Nut Rock)

Sedimentary Concretion (specifically an 'Omarolluk' or Omar)

AI-generated identification · may be incorrect

Hardness: 6-7 (Mohs scale), Color: Grey to brownish-grey with tan interior, Luster: Dull/Earthy, Structure: Fine-grained sandstone with deep hemispherical pits, Specific Gravity: 2.5-2.6

Identified May 22, 2026More sedimentary →

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Physical properties

Hardness: 6-7 (Mohs scale), Color: Grey to brownish-grey with tan interior, Luster: Dull/Earthy, Structure: Fine-grained sandstone with deep hemispherical pits, Specific Gravity: 2.5-2.6

Formation & geological history

Formed in the Proterozoic eon (roughly 1.8 billion years ago) in the Belcher Fold Belt of Hudson Bay. These rocks are created when mineral cements precipitate around a nucleus within sediment, forming a hard mass that resists erosion.

Uses & applications

Primarily used as an index clast for tracking glacial movement during the Pleistocene epoch. They are popular among collectors as 'curiosity' rocks or 'nut stones' due to their unique shapes.

Geological facts

These specific pitted rocks are called Omars. The pit is formed by the erosion of a more soft, less-cemented mineral inclusion (often calcite or a carbonate mineral) that washes out once the rock is exposed.

Field identification & locations

Identify by looking for a smooth, rounded grey sandstone cobble with a distinct, perfectly circular pit or 'cup'. Primarily found in glacial drift areas across central North America and Canada.