I know I said I would reveal the secret in the comments, but since you may not remember to go back and look, I'll post the answer here.
The purple round thing is a bowling ball covered by purple glass beads. It's actually garden art made by HRH The Queen Mother, but since she first told me it was a bowling ball, that's what I see when I look at the picture. I'm thinking it will look better when I see it in real life.
Side note: The Queen Mum is not a bowler.