We had both breeds in a single order, and we sorted them by size until they were about 4-5 weeks old. By that time, there was a pretty distinct difference in their weights, since the rocks were so much heavier. When we sold some of them at 5 weeks, we informed all buyers that we had made our best guess of which was which, but that we couldn't guarantee it. Now that they're adults, it's very obvious in their size.