sedimentary
Omarolluk
Omarolluk Formation greywacke concretion
Hardness: ~6-7 (Mohs), Color: dark grey to black, often with lighter hemispherical indentations, Luster: dull to earthy, Crystal structure: clastic (fine-grained), Cleavage: none, Specific gravity: ~2.6-2.7
Identified More sedimentary →
Explore Omarolluk in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: ~6-7 (Mohs), Color: dark grey to black, often with lighter hemispherical indentations, Luster: dull to earthy, Crystal structure: clastic (fine-grained), Cleavage: none, Specific gravity: ~2.6-2.7
Formation & geological history
Formed during the Proterozoic Eon (approx. 1.8 billion years ago) as carbonate concretions within siliceous greywacke in the Belcher Islands of Hudson Bay, Canada. Later transported southward by glaciers during Pleistocene ice ages.
Uses & applications
Primarily collected by rockhounds and geologists as glacial erratics and curiosities. Sometimes used in educational settings to demonstrate glacial transport.
Geological facts
Often called 'Omars', these stones are famous for their distinctive hemispherical pits, which are the remains of weathered-out carbonate concretions. They are excellent indicator erratics, helping scientists trace ancient glacier movements from Hudson Bay across North America.
Field identification & locations
Identified by their dark grey matrix and characteristic round, lighter-colored pits or holes. Commonly found in glacial deposits, riverbeds, and beaches across central North America, particularly around the Great Lakes region. Tip: look for the smooth, weathered surface with distinct dimples.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Quartz pebble
Silicon dioxide (SiO2)
sedimentary
Fossiliferous Concretion
Septarian Concretion containing Brachiopods/Crinoids
fossil
Generic Pebble / Rock Fragment
Lithic fragment / Weathered rock
sedimentary
Sedimentary Sandstone
Arenite (Sedimentary Rock)
sedimentary
Banded Siltstone / Mudstone Pebble
Siltstone / Argillite
sedimentary