There's a big rock, north of North Railyard and west of the railroad track that has a diamond case.
Use the glider found west of Cattle Xing and south of Petro Sahel, fly around the big mountain then across the tracks.
I've also found that you can jump off the glider, you don't have to land it. Just use the glider again while flying, use the interact key (e) and you will let go of the glider and fall to the ground. You should do this when near the diamond case.