I'm actually currently working on this issue for a presentation at the upcoming CAA conference in Oslo! Specifically, we're comparing different coatings as well as different scanning methods (i.e. structured light vs. photogrammetry) for use with obsidian artifacts, which are frustratingly both shiny and clear.
Normally, for lithics I use developer spray. This is what I used to coat my core models from Les Cottés, and what my colleagues and I published in our recent paper in Advances in Archaeologial Practice (you can download the article from my website here). I would definitely not advocate using this on ceramics, or anything that is porous. It needs to be cleaned off with water. Furthermore, it contains some acetone. Again, not a big deal for stone tools (although we commonly do have to re-label artifacts after use) but possibly a big problem for most other things.
Recently, we've had some good luck with dulling spray (see model below). However, this stuff only works with structured light scanning, and not photogrammetry. We also wouldn't really recommend using any of these coatings on objects that may have residues, or that will be subject to any type of chemical analysis.
Experimental Obsidian Point
by Samantha Porter
The next thing we'd like to try is cross-polarized photography. Anyone here have any experience with that?