Geologic Egg

One man's SWAG would be: the rock is a tuff, a welded tuff, a water-laid tuff, or a fine grained sandstone. While in place, and buried beneath younger sediments and volcanics, the brittle rock was fractured extensively by tectonic or other forces and developed joint sets including 90 degree angle sets. Introduction of groundwater circulation followed and the fractured rock developed a rind from chemical weathering which penetrated from the joint surfaces into the center of the rock. If the guess on rock type is off the mark, the diagnosis of a weathering rind is still probably on target. In the exceptionally humid Southeast, we see weathering rinds a lot, and the pictured hand sample sure looks like one.

Foy

I agree with Foy. Weathering rind in an ash flow tuff. There is a lot of volcanic rock in eastern Oregon, if that is where it is from.

I found it here at Fort Sagebrush. The previous owner had piled a bunch of rocks away from the building site.

Thanks for the ID!