Rocks and Minerals Information Sheet R005
These are often referred to as "fossil tree ferns", but dendritic crystal growth is what they actually are.
The same type of growth is also seen in snowflakes. The word Dendrite comes from the Greek Dendron meaning tree. And it is also a term in neurology, referring to the tree like shape of neurons.
In paleontology these crystal forms are sometimes mistaken for fossilised plants. The ones on my site are Manganese dendrites on a limestone bedding and are from Solnhofen in Germany.
They create wonderful patterns resembling mini-landscapes. The patterns form when water which is rich in Manganese and Iron flows into fissures in the layers of limestone and other rocks depositing dendritic crystals as the liquid flows through.