I have seen trigons inside a diamond on a surface breaking cleavage. I took pictures, but both the photos and the negatives have long since been lost. It was a very interesting thing to have seen, too bad it happened before the advent of digital cameras. (Yes children, there really was such a time...)
As to your actual question, they can appear on naturals and indented naturals. they are normally observed on the surface of the rough, so it is not necessary for the natural to be indended to be seen, only that that part of the surface not be polished away.