1) Dross 
elements in the crystal structure that are foreign to the basic chemical composition of the gem.
<radiobiology> Atoms (or ions) of unwanted elements in the plasma, which tend to degrade plasma performance, and in the case of fusion plasmas tends to inhibit fusion (poisoning the reactor). ... See: poisoning. ... (09 Oct 1997) ... 
pollution or matter that alters the purity of a substance
Elements or compounds whose presence in a material is undesired.
Any nongrain material contained within a shipment that could hinder the processing of a grain or detract from its end value.
Elements or compounds whose presence in a material is undesired.