Wood is clearly not the best material for an archer’s thumb ring. It might splinter and it might break with little or no warning. The video below shows a hammer test on a thumb ring I’m pretty certain is cumaru. I’ve come to trust cumaru less than other ironwoods, but this little guy was super tough and took quite a beating before breakage. Since I started crafting thumb rings, I’ve noticed the same species of wood can vary greatly from one piece to another. One good thing about cumaru so far, is that it seems to crack or break slowly. Whether that’s the case at all or if depends on moisture content, I’m not sure. For modest use and especially for lower poundages, eg <45lbs, they seem to do well. For consistent use over 45lbs, I suspect the risk of breakage is formidable, but this largely depends on the individual piece and species.