Owyhee picture jasper is found in southwest Idaho and (I think) over into Oregon. I have slabbed and cabbed some beautiful pieces from the Owyhee Mountains.
The horse chert is nice! I had never heard of it before seeing this thread. Will have to look into where it is found.
On a side note, I have picked up some pieces of chert on Lake Superior shoreline that polished up nicely in the tumbler. Haven't found one that I have thought would make a nice cab---->nothing like the photo's of the horse chert.
Thanks a lot. Now I have to find some to add to my (already too large) collection of rocks to slab and cab.