Very, very exposed sea cave. Black basalt outside, smooth volcanic tuff inside. Fairly good quality rock. So far I have messed around on two routes. The first is a traverses up and right along the eastern outside edge of the cave. You can get very high fairly easily, but tougher variations undoubtedly exist. Inside the cave is a perfect finger slot, fully overhung. This goes maybe 40 feet up then ends. I can't imagine that it is possible to top out on this route, but its fun while it lasts.
Northwest tip of Outer Brass, cave faces north. Conditions have to be extremely calm to climb here. Given those conditions, however, you can drive a boat into the cave and start dry on the route inside.