Amalgam discolorations are caused by deposits of amalgam filling in the dentin. As seen in the video, an amalgam filling breaks into small pieces while being removed. Below you can view partial caries and partial amalgam discoloration.
The general rule is to preserve as much tooth structure as possible. Whether or not the discolorations decrease the adherence between ceramic inlays and the dentin is subject to controversial scientific discussion; moreover, the difference between the various glue combinations should not play a role in actual clinical practice.Amalgam Discoloration