This video by Karen Silton describes how to use thinset to attach mosaic tiles on a gazing ball.
You can use any kind of tile on the gazing ball, creating a pattern or a design on the ball. It’s important to have something like the thinset on the ball so that the tiles don’t fall off when it gets wet from sitting outside. A styrofoam ball can be used under a layer of the thinset, which is similar to cement, in order to decrease the weight of the ball. This makes it easier to lift after the tiles are in place.
This informative video is about 15 minutes in length, it gives several details of how to mix the thinset and apply the tiles to the surface.