Decoration
Index
Dislocation lines act as a trapping location for impurity atoms in a crystal. A method of locating dislocation lines in transparent materials entails "decorating" them with such impurity atoms. These particles scatter light and the defect can then be viewed with a high powered microscope. 

In silver chloride, the material may be decomposed to yield metallic silver when exposed to light in the fundamental absorption edge.  The interstitial silver migrates to dislocation lines and may be seen by viewing the crystal in non-photoactive light. The top photograph shows a dislocation network in silver chloride made visible in this way. Materials that do not photodecompose may have their dislocations decorated with impurity atoms. The lower photograph shows dislocations in KCl decorated with silver impurities.

From: Amelinckx, "The Direct Observation of Dislocations," Academic Press (1964)