I've made a few sphere's and Slow asked me to post and explain how I did them.For the blue one I used an exersize ball. I did three layers and used drywall mesh and fibres for addition strength. I added some fibres to my concrete mix and did three thin coatings by applying the cement like thin hamburger patties with strips of drywall tape in
between for additional strength. I left a rough opening on the top, let the air out and removed the ball. I've painted the inside black, and left a small hole in the back to run electric and usually use it with a mister inside. You could mosaic both in and outside if you wanted.
I have another one that I started to make and just couldn't
get the cement mix to cooperate with me. I ended up just putting three balls of cement on the top of it and when it cured I upended it and turned it into a planter, although again this could be mosaiced.Both of the above I painted shades of blue.
I also made a concrete solid sphere and mosaiced it. I use old glass light globes, fill them with concrete and let them cure then break the glass off.I've made a number of bubblers using this method, just inserting a piece of pipe or tubing in your glass globe while you add the concrete mix around it.