There is a well-known limb cast collection area at the South Fork of the Crooked River.  This is for the first site.  The limb casts were formed as such according to this website:

"Over time, the tree’s wood material eventually decayed, leaving perfect molds of tree limbs and roots. These forms filled with quartz and other minerals producing some very beautiful agates in the shape of tree limbs".

Here is a little instruction:

"To find limb casts it is necessary to dig a little, as they tend to lie below the surface at an average depth of 18 inches. They are also occasionally found in streams and on the surface, though not as frequently as they used to be"

I was unable to locate the location on Google Earth, but it is a well-known locality (search google for it). According to this website, here are the directions from Prineville:

"To get there (from Prineville), follow Hwy 380 (Combs Flat Road headed toward Paulina) to mile post 51, turn right on Congleton Hollow Road. After 3.9 miles take a left on the road and proceed 0.4 miles, and park."