One of the things to keep in mind is how highly reflective zirconia really is. Plus we tend to photograph it against a black background or mirror. Add to that, we're using the flash - possibly set to ETTL/TTL, and maybe we're using a photo box - maybe not. That is contrast city! With emax, you could probably get very good results... not so much with zirconia.
Now try to think outside that box. How about shooting against a neutral gray or off-white marble? How about turning the flash waaaay down on manual settings and use small white pieces of paper to bounce around what little light you are using? (fold white cardboard into 4"x 2" L shapes and position them opposite the light source) Shooting inside a photo box is a good thing, but using the same principles of where the light is coming from and how you want to "fill" the scene, you can move these little pieces of "reflectors" to achieve more dramatic effects without blasting the zirconia with direct or even directional diffused light.
Zirconia is a problem - but that should also make us think about how we keep it from glaring in the mouth. All the same principles of diffusion have to be taken into account as we apply in making them look natural. The difference? When we photograph them, it is often the same thing as when the doctor is taking the shade with his overhead spotlight on. Don't do it!
If you've seen some of the foldable reflectors pro photographers use, you would see gold, silver and white. We also use black panels to shape the direction or cut out parts of light. Its all about controlling the light that is spilling all over the scene. Buying strips of gold lame material and using it like the white folded paper will warm the scene and reflect light around - though you may need to tweak the white balance. Silver or aluminum foil might be better - but could introduce gray.
Instead of using the flash, (if you're using a light box) try using incandescent or florescent lights. You will need to color white balance. But possibly these lights won't be so intense. Light is light - the only difference is its color shift.
Zirconia is a dead material - light only the overlay porcelain if there is any. The only way to do that is not pour so much light in as it excites the zirconia to nuclear reflectivity. If you find the image gets too dark, change the shutter speed or open the aperture to compensate. Remember, underexposed images when brought up in post will have noise introduced - which makes the image funky. My earlier suggestion to slightly over-expose an image really wouldn't apply to zirconia - though if shooting in RAW and then doing post and its fairly close it won't hurt you too much. Just remember that when you turn down the light source to control reflectivity, you have to compensate with the camera settings to get a properly exposed image.