I made this orb from hypertufa and then treated it with Iron Sulfate to give it a rustic look. It reminds me of an old wrecking ball.
Here's a photo tutorial of what I did.
Please, always use safety gear: chemical resistant gloves, safety glasses and dust mask.
Step1 - wrap with plastic wrap
Step2 - tape with cement tape
Cement Step3 was actually two steps: I covered one half of the 16" schoolyard ball with cement and let it dry before flipping it over and repeating the process on the other half. I made sure not to cement over a 5" radius from the hole so I could deflate and remove the ball.
Step4, "rust," is when I sprinkled iron sulfate, sprayed the whole thing with water and let it sit for about 4 hours to "rust." Iron sulfate will stain almost everything so be sure to protect surfaces you don't want stained.
Step5-deflate with a pump needle. Remove the ball and plastic wrap.