Dendritic Agate Identification Guide

What Dendritic Agate Looks Like

Dendritic agate is a translucent to milky-white or pale-gray chalcedony (microcrystalline quartz) carrying dark, fern-, tree-, or moss-like inclusions called dendrites. The dendrites are usually black, brown, or reddish and are mineral deposits of manganese and iron oxides (pyrolusite, psilomelane, goethite) that crystallized along tiny fractures, not fossilized plants.

Key visual cues:

• Body: colorless, white, or grayish, often glassy and waxy where polished.
• Luster: waxy to vitreous.
• Transparency: translucent; you can usually see depth into the stone.
• Pattern: branching, two-dimensional "plant" growths that often sit in flat planes inside the stone.
• Banding: true agate banding may be weak or absent — many "dendritic agates" are technically dendritic chalcedony.

Step-by-Step Field-ID Checklist

1. Check translucency. Hold to light. A genuine dendritic agate transmits light through its body; opaque pieces are more likely dendritic jasper.
2. Inspect the dendrites with a loupe. Real dendrites branch fractally and fade into the host. Painted or surface markings sit only on the surface.
3. Test hardness. It should scratch glass and resist a steel knife (Mohs ~7).
4. Feel the surface. Polished agate feels glassy and cool; it takes a high polish.
5. Look for a waxy conchoidal break on any chipped edge.

Diagnostic Tests

• Mohs hardness: 6.5–7. It scratches glass and steel.
• Streak: white (the chalcedony body); dendrites alone may streak dark brown/black.
• Fracture: conchoidal, no cleavage.
• Density: ~2.58–2.64 g/cm³, typical for quartz.
• Acid: inert to dilute HCl (distinguishes it from carbonates).
• Magnetism: none.

Common Look-Alikes and How to Tell Them Apart

• Moss agate: inclusions form clustered, three-dimensional mossy clouds (often green chlorite or iron) rather than flat, branching dendrites. Dendrites are tree-like and planar; moss is blobby.
• Dendritic jasper: opaque, not translucent; same dendrites but the host transmits no light.
• Dendritic opal: softer (Mohs 5.5–6.5), lighter, may show play-of-color or a more porcelain look; opal will not scratch glass as readily.
• Dendritic limestone (Solnhofen-type): fizzes in dilute acid and is much softer (Mohs ~3); dendritic agate is acid-inert and hard.
• Manganese dendrites on shale: the host is soft, dull rock that breaks easily and may fizz; agate is hard and glassy.

Where It Is Found

Dendritic agate forms in cavities and fractures of volcanic and sedimentary rocks where silica-rich groundwater deposits chalcedony and manganese/iron-bearing solutions infiltrate microcracks. Notable sources include India (Deccan Traps), Brazil, Kazakhstan, the United States (Oregon, Montana, the Mojave region), and Madagascar. Field collectors typically find it as waterworn nodules in gravels, in basalt vesicles, or weathered out of volcanic flows.