This pattern created by Nature, evolved in Jurassic period. Mineral rich water with high concentration of metals, iron and manganese, migrated into microscopic cavities between the layers of limestone created these patterns that are often mistaken for fossilized fern. The Jurassic limestone has been created about 160 mil. years ago.
Found in Solnhofen, Germany
See also Richard Weston's wonderful blog with a comment on our dendrites
We also offer Solnhofen dendrite clocks and pendants made of this limestone.