I did not find one until way after I had found his summon area. The summon area is like the size of a skyscraper in a football field. The where to find that stones are like the size of a furling and have maybe one or two 'shaped' or 'worked' rocks around them. Just go looking and you will stumble over one or the other, but the summon alter is the easier one to see -- if you get near it, you will not miss it.