Dendritic Opal Identification Guide

What Dendritic Opal Looks Like

Dendritic opal is a common opal (non-iridescent, hydrated silica) hosting dark, branching, fern-like dendrites of manganese and iron oxides. The body is usually white, cream, beige, or pale green, often with a porcelain-like or slightly waxy look. Most dendritic opal shows no play-of-color — it is "common" or "potch" opal valued for its scenic dendrites.

Key visual cues:

• Body: white, milky, cream, tan, or soft green; smooth and porcelain-like.
• Luster: waxy to subvitreous, sometimes slightly resinous.
• Transparency: translucent to opaque.
• Pattern: black to brown tree-, moss-, or fern-like dendrites.

Step-by-Step Field-ID Checklist

1. Assess the body feel. Opal looks softer and more "creamy" than glassy agate.
2. Test hardness carefully. Opal is softer (Mohs 5.5–6.5) — it may be scratched by quartz and a hard steel file, and it will NOT readily scratch glass the way agate does.
3. Examine dendrites with a loupe for fractal branching that fades into the stone.
4. Check for play-of-color by tilting under light; common dendritic opal lacks it.
5. Inspect for crazing — fine surface cracks common in opal but absent in agate/jasper.

Diagnostic Tests

• Mohs hardness: 5.5–6.5 (softer than agate/jasper at ~7). This is the key separator.
• Streak: white.
• Fracture: conchoidal; no cleavage.
• Density: ~1.9–2.2 g/cm³ — noticeably lighter than agate (~2.6). It often feels light for its size.
• Acid: inert to dilute HCl.
• Water/hydrophane test: some dendritic opal is hydrophane and may stick slightly to the tongue or change clarity when wet.

Common Look-Alikes and How to Tell Them Apart

• Dendritic agate/chalcedony: harder (scratches glass), denser, and glassier; opal is softer and lighter. A density check or a glass-scratch test settles it.
• Dendritic jasper: opaque and quartz-hard; opal is softer and often more translucent and porcelain-like.
• Dendritic limestone: fizzes in acid and is much softer; opal is acid-inert.
• Moss opal: the inclusions are cloudy, three-dimensional clusters rather than flat branching dendrites.
• Howlite/magnesite (dyed): softer still (Mohs 3.5) and fizzes (magnesite) or has porous veining; lacks true dendrites.

Where It Is Found

Dendritic (common) opal forms from low-temperature silica gels in volcanic and sedimentary settings, with oxide-rich water depositing dendrites in cracks. Notable sources include Kazakhstan, India, Australia, Madagascar, Peru, and the western United States. It is often found as seams, nodules, and replacement masses in weathered volcanic terrain.