I believe the margin of error of the current generation of RealSense cameras is around +- 5%. This may have improved further for the new super-powerful D415 and D435 models that are launching in September. Detailed specifications for those models are not available yet.
Reflective surfaces such as glass, jewelry and spectacles can make it harder for the camera to read those surfaces. It is possible to make it easier for the camera to perceive these surfaces by applying a powder or spray to the surface.
This snapshot from page 17 of the ZR300 data sheet may provide the depth resolution information that you seek.