Specimen Mountain: Allophane & Calcite Rockhounding in Larimer County, Colorado

Plan your visit

Permission required before collecting

County

Larimer County

State

Colorado

Coordinates

40.44276, -105.80841

Coordinates are approximate. Confirm the exact ground in the RockHoundR app, where land overlays show which agency manages the area.

Land & collecting status

Permission required before collecting

Requires landowner or agency permission before any collecting. Do not enter without explicit authorization.

Public-land rules vary by agency, season, and field office. The RockHoundR app pulls live BLM, USFS, NPS, and tribal overlays so you can see exactly which agency manages the ground at this spot.

Geology at Specimen Mountain

This spot sits on Rhyolite welded tuff, mapped as extrusive igneous (volcanic) that formed roughly 28 to 23 million years ago. Volcanic ground like this is where agate, jasper, and other chalcedony typically form, filling old gas pockets and fracture seams in the flow rock.

Mapped unit

Rhyolite welded tuff

Age

28–23 Ma (Late Oligocene)

Rock types

extrusive igneous (volcanic)

Elevation

11,980 ft

Geologic mapping via Macrostrat and the underlying state geological survey maps (CC BY 4.0). Bedrock at the surface can vary within a mapped unit.

Sources & verification

Spot details combine the public RockHoundR location dataset, normalized mineral labels, agency land-status checks in the app, and community submissions. Coordinates are approximate until verified in the field.

• BLM Colorado rockhounding rules
• Colorado Geological Survey
• Pike and San Isabel National Forests
• Community submissions reviewed against published agency rules