Moss Opal Identification Guide
A field guide to identifying moss opal by its silica body, waxy luster, and the dendritic mineral inclusions that create its mossy patterns.
Read the full Moss Opal encyclopedia entry →
What Moss Opal Looks Like
Moss opal is a common (non-precious) opal whose translucent to milky silica body is threaded with dark, branching dendrites of manganese and iron oxides that look like moss, ferns, or trapped vegetation. The base is usually whitish, grey, cream, or pale bluish, and the inclusions range from green and brown to black.
- Color: pale milky base with green/brown/black mossy or fern-like inclusions
- Luster: waxy to subvitreous (never metallic)
- Transparency: translucent to nearly opaque
- Habit: massive, amorphous nodules and seams; no crystals
- Fracture: conchoidal, sometimes with a slightly greasy feel
Moss opal does not show a play-of-color (the rainbow flash of precious opal); its appeal is purely the inclusion pattern.
Step-by-Step Field ID Checklist
- Confirm it is amorphous silica: glassy to waxy, no visible crystals or grains.
- Look for 3-D dendrites suspended inside the stone, not painted on the surface.
- Check for the absence of fire/play-of-color (separates it from precious opal).
- Test hardness: it should scratch glass only weakly (around 5.5-6.5).
- Note the low heft — opal feels lighter than agate of the same size.
Key Diagnostic Tests
- Mohs hardness: 5.5-6.5. Softer than agate/chalcedony (7); a quartz point will scratch it.
- Streak: white.
- Cleavage/fracture: none; conchoidal fracture.
- Density: low, SG ~2.0-2.2 (notably lighter than chalcedony at 2.6).
- Acid: no reaction (it is silica, not carbonate).
- UV: some opal fluoresces green or white under shortwave UV.
Common Look-Alikes and How to Tell Them Apart
- Moss agate: the classic confusion. Moss agate is chalcedony (Mohs 7, SG ~2.6) and will scratch glass cleanly and resist a quartz point; moss opal is softer and lighter. Agate also tends to be glassier and more translucent.
- Dendritic agate/dendritic opal: dendritic forms have the same inclusions but in clear, ribbon-like layers; "moss" implies denser, three-dimensional sprays.
- Plume agate: plumes radiate from a point and are usually colorful mineral plumes, not dark oxide moss.
Where It Is Found
Moss opal occurs in silica-rich volcanic and sedimentary settings worldwide, notably the western United States (Oregon, Nevada, Idaho), Australia, Indonesia, and India, typically as nodules and seam fillings in weathered volcanic rock.
Frequently asked questions
How can you tell if it's real moss opal?
Genuine moss opal is an amorphous silica with a waxy luster, no play-of-color, a hardness of only 5.5-6.5 (it is scratched by quartz), and a low density around 2.0-2.2. The dark mossy dendrites should be suspended in three dimensions inside the stone, not printed on the surface.
What is the difference between moss opal and moss agate?
Both carry mossy mineral dendrites, but moss agate is chalcedony with a hardness of 7 and density near 2.6, while moss opal is hydrated silica that is softer (5.5-6.5) and noticeably lighter. A quartz scratch test separates them quickly.
Does moss opal have fire or play-of-color?
No. Moss opal is a common opal, so it shows no rainbow play-of-color. Its only optical feature is the green-to-black mossy inclusion pattern within a milky silica body.
What does moss opal look like?
It looks like a milky, waxy, translucent stone with fern-, moss-, or tree-like dark green, brown, or black branching inclusions frozen inside it.