sedimentary
Sandstone Concretion (Omar or Nut Rock)
Sedimentary Concretion (specifically an 'Omarolluk' or Omar)
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 More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
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.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone
Arenite
sedimentary
Sandstone (Quartz-rich)
Arenite (SiO2)
sedimentary