A nice dendrite plate from the famous Solnhofen Limestone.
 
Dendrites form in rocks when mineral-rich water seeps into cracks and fractures.  Following water evaporation, minerals such as iron and magnesium crystallise. These crystals then grow in fascinating patterns, such as the example of this specimen. 
 
Location: Solnhofen, Germany.
 
Measurements of the whole piece: 3.15 x 2.32 inches