There's a prior thread here from perhaps a month or so ago in which someone asked about cutting a walnut stump he had into gunstock blanks. I answered there with my own experiences of doing the same thing and ending up with about 35 very good to excellent 3"+ thick blanks. That took place a few years ago, and I have since used some of the wood and am very happy with it. I still have the remainder. A few others also gave their experiences, including Pete, our resident wood guru. The roots may not offer much to you, unless they are exceptionally large roots, but a large walnut stump certainly will. The roots may well make knife scales, pen blanks, or such. All the wood will need to be properly sawn, the ends sealed, and the wood dried properly before it can be used, but you'll have some nice wood.