Crook County, Oregon, calls itself the rockhound capital of the world, and every year thousands of collectors come through Prineville with maps and shovels, some from as far away as France, Japan and New Zealand. Eagle Rock and Congleton Hollow are two of the free collecting grounds the Forest Service names on the Ochoco National Forest, the first for agate, the second, out toward Paulina with Dendrite Butte, for limb casts.
The stones come from old fire. Some 40 to 50 million years ago a band of volcanoes erupted across the region, and superheated, silica-rich groundwater circulating through the rocks filled their cracks and voids with quartz, calcite, cinnabar, agate and jasper. Today the eroded Clarno and John Day formations expose layer after layer of that material. Local agate is a translucent quartz, usually white to colourless and often banded; the prize pieces carry swirled grain, like angel wing or tube agate, or plumes, moss and dendrites that look like tiny trees trapped beneath the surface. Often you cannot tell how good a stone is until it is cut or polished, and many rockhounds count that as part of the fun.
Limb casts are stranger. When hot volcanic ash buried pieces of wood, the heat burned the wood away and left hollow moulds in the shape of the branches; later, agate filled the moulds, and the best specimens still show bark, knots and the texture of the grain on their surface. Casts with a green or pink hue are the most highly prized.
Eagle Rock lies about an hour from Bend, southeast of Prineville, and a large, imposing rock marks the turnoff, though one writer for the Bend Source admitted failing to see any eagle in it. The writer's family dug there and filled their buckets with stones showing bits of agate, past an old school bus left rotting on the way in.
The rules are simple. Forest Service and BLM land share these hills, collecting is for personal use only and nothing may be resold, small amounts carry no fee, and you may dig with a shovel but not with machinery or explosives. As Ochoco geologist Carrie Gordon told The Bulletin, some of the best pieces turn up simply walking the slopes in spring.
Jasper, agate, limb casts
Other places on the map within a couple of hours.
