Dendrites
Friday May 2, 2008
They look like fossils, but dendrites are purely mineral growths whose fractal, arborescent habit gives them an organic appearance. Instead, they're generally made of psilomelane or pyrite. Museum specimens and expensive moss agates can be spectacular, but if you keep your eyes open you'll see dendrites in humbler guise, like I did in this sandstone boulder at the Dinosaur Ridge headquarters building.Dendrite — Geology Guide photo
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No joke—dendrites are where you find them. Last August I found some beautiful dendrites (dark black in color—maybe manganese of some sort?) in a fine sandstone near Los Banos. The stone was mixed in with some conglomerates from the Moreno Formation.
The dendrites are small but quite vivid.
Ooops—should have read the link to psilomelane. They ARE manganese, of course.
You can also often see beautiful Mn and Fe oxide dendrites in serpentinites.
Oops the URL didn’t turn out right on the last comment.