Dendritic agate is a whitish-gray or colorless chalcedony with tree- or fern-like markings known as dendrites. The dendrites in dendritic agate are iron or manganese inclusions, usually brown or black in color. Though they appear organic due to their fern-like structure, they are actually inorganic; they are formed at fine fracture surfaces through crystallization of weathered solutions of neighboring rock.
An antique moss agate brooch, Fabergé, workmaster Michael Perchin, St. Petersburg, circa 1890. The hexagonal façade composed of three moss agate sections within rose-cut diamond borders, set with rubies and gems, 56 standard, length: 3.7cm.
Fractal mineral space by Pascal Petit. Incredible colors! Igneous rocks formed from lava flows of the Archaean era (>2,700 million years ago) are often found to contain disequilibrium-textured crystals characterized by spherulitic, branching or dendritic morphologies that occur in layers near the flow surface.