You could add extra bonding material under the thinner pieces to raise them up to the same height as the thicker pieces. Otherwise the finished project will be difficult to grout and will have a patchy appeareance. Grout will want to flow over the thinner material and will be a bear to remove.

I use different thickness all of the time, sometimes I do it to create a "look" I'm going for, I haven't done a bowling ball for a while now...have decided to switch to making my own cement balls(out of light globes filled with concrete then after curing,smash the glass off)as they will hold up in the freezing weather better, but have a few to mosaic, just been busy with other stuff so they are at the back of the Project List(

ohhh, calamity, I like that idea - there are lots of cool light globes out there......

Kelly- almost everything I do has various thicknesses and media. It is all a **# to grout but take your time and it can be done.

Add glass or plastic lid or any thing under to raise you glass up.I do it all the time.