What is a faden crystal in mineralogy ?

Faden : definition

In certain particularly transparent crystals, the term "faden" is used to designate an opaque, usually milky, rectilinear zone. This "faden" mostly translates the presence of a myriad of tiny fluid inclusions (less than 100 µm), witnesses of the original hydrothermal fluid trapped during the formation of the crystal.
 
The rock crystals of certain alpine slits constitute magnificent "faden quartz" : those of the Lauzière and Mont-Blanc mountains, for example, are world famous.