This stone is probably some of the oldest rock in my collection, clocking in at about 2 billion years old. It's a banding of jasper, hematite and magnetite that dates to the genesis of life on planet Earth. From what I'm told it is quite plentiful in Wyoming, so much that homeowners nearby have used it for landscaping. I found mine at a bucket sale in Tempe.
Here's an interesting webpage that describes it in more detail.