RE: Aventurine glass
Sparkle side up, if “up” is going to be your most prominent surface.
If you cap it with clear, there will be some “depth” to the piece. If you put the aventurine on the clear base, you get a “flatter” looking surface, but it may have more sparkle.
All depends upon the look you are going for. I would fire a couple of small test pieces.
I find aventurine a bear to cut.