When I first saw Surface, the thing that blew me way was the use of transparent tiles as ID tags.
Over the course of development, Microsoft stopped pushing this idea, likely because the acrylic pieces create too many false touches. This seems short sighted to me, so I did a some materials research and developed a range of options that work pretty well.
1. Choose a material with a more appropriate refraction index.
2. Use an IR reflective coating.
Here is an IR reflective paint:
3. Scuff the acrylic to turn it into a screen. We have had good success with this.
If you machine this really well, you can also get contact feedback from on top of the artifact. It feels REALLY nice.
A word to the wise: Messing around with interactivity on top of lenses can rapidly get you into mathematics that you might rather avoid.
4. You can mask or etch a frame around the artifact. Laser etching is a nice,inexpensive way to do this. You can also create a partial screen on top of the surface. This is a pretty cool effect.
Thanks to Staph for reminding me about this.