Spectrum System 96
All System 96 can be fused with either side facing up. I do have a few preferences.
UR Plum Opal will get a surface similar to devit if you fuse it with the rough side facing up. You should always find the smoothest side and fire it face up.
The opal arts and spirits have the design layer on one side of the glass. Determining which side has the design layer is similar to checking clear dichroic glass. Once you find the design layer, you want to be consistent in your design so that the pieces look right together. If you want the design to look like it is clear capped, make sure that the design layer is on the bottom. I usually prefer for the design layer to be on the top. When you cut opal art or spirit, you will usually have more success if you cut it on the smooth side, opposite from the design layer.