Omars are dark siliceous greywacke stones, from pebbles to boulders, pitted with rounded, often deep hollows, left where carbonate concretions dissolved out of the rock. The pits are usually between half an inch and 3 inches across. They are glacial erratics from the Proterozoic Omarolluk Formation of the Belcher Islands in Hudson Bay, carried more than 700 miles by the Laurentide Ice Sheet. Because the source area is so small, geologists use their spread to reconstruct how the ice sheet flowed. They resemble Keweenawan basalt in color and texture but are sedimentary.
Is this omar?
3-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Read the textureLook at a fresh break with a hand lens. Texture: Rounded pebbles to boulders with hemispherical pits where carbonate concretions dissolved out. Grain size: fine-grained.
- 2Pick out the mineralsComposition: Siliceous greywacke with 10 to 40 percent rock fragments, volcanic clasts and spherical carbonate concretions. Identify the two or three most abundant grains.
- 3Match the color rangeCompare against the expected color range: dark gray, gray, tannish gray.
Often confused with
Omar vs. its common look-alikes, and how to tell them apart in the field.
All properties
- Uses
- Scientific research
- Host rock
- Glacial till and outwash gravel; eroded from the Proterozoic Omarolluk Formation, Belcher Islands, Hudson Bay
- Texture
- Rounded pebbles to boulders with hemispherical pits where carbonate concretions dissolved out
- Grain size
- fine-grained
- Composition
- Siliceous greywacke with 10 to 40 percent rock fragments, volcanic clasts and spherical carbonate concretions
Where rockhounds find omar
5 mapped spotsClassic worldwide localities
- Belcher Islands, Hudson Bay, Canada (source formation)
Field-hunting tip
Where it forms: Glacial till and outwash gravel; eroded from the Proterozoic Omarolluk Formation, Belcher Islands, Hudson Bay. In the U.S., the densest reported localities are in Michigan. Start trip planning there.
More omar photos
Common questions
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Find omar on the map
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