I use all parts of the antler for pens. Some of these have been up to almost two inches in diameter. The inside of the larger diameter sections is very porous, so I don't use it for pens that have thin wall thicknesses. (If a piece comes out, the brass will show, and filling it will disrupt the patterns of the colors. It looks like a pothole that has been filled.)
When you are about 1/16" proud of the bushings, start slurry sanding. Continue till all voids are filled. When you get down to the bushings, using plain sandpaper, there should be no voids or pits remaining.
The colors inside are often spectacular. While all antler pens are attractive, it is my experience that those with the most colors are those the customers find most interesting. Browns, yellows and purples are not unusual.
Just my experience.
Joe