Some of the folks over on the Hypertufa forum have made concrete balls. Have you checked over there?
I know the forum name says hypertufa but it's also a creative concrete stuff play room. Fun! Fun! Fun!

I have done mosaics with flat sided marbles on a rubber ball and it turned out good.

Here is a link that might be useful: mini gazing ball

What an interesting idea. I think the hypertufa folks were using beach balls and then deflating and pulling them out. I wonder what happens to the ball when it goes from 60 degrees to 10 below zero in a 24 hour period? This would worry me. Anyone have any experience?
Craftygardener, I like yours a lot.
kathy

I think it would be much less work to make one on a bowling ball . It's already a solid sphere - you wouldn't need the concrete.

Hi Kathy,
I've found that temperature swings don't affect the concrete balls. So far all I've had to be watchful for is freezing temps. I seal my concrete creations quite well but I still keep an eye on them. Fortunately in my area we don't usually get too cold.
I've used beach balls, basketballs and gym balls (those rubber ones we used to play dodge ball or kickball). I have a friend that uses balloons and acrylic concrete. It's messy but it's fun!
Hmmmm sounds like my cooking.......

YOu don't get too cold ??? Now I'm jealous .... LOL

crafty gardener...did you prep your rubber balls with anything prior to putting on your gems? Is there a specific way to add gems, etc to any of the different types of balls...top down...sideways, etc?

Thanks

Rosemarie