Menilite Opal Identification Guide
A practical guide to identifying menilite (liver) opal, a brown nodular common opal, and distinguishing it from flint, chert, and precious opal.
Read the full Menilite Opal encyclopedia entry →
What Menilite Opal Looks Like
Menilite opal (also called liver opal or menilite) is a variety of common opal — amorphous hydrated silica (SiO2·nH2O) — that lacks play-of-color. It forms dull, opaque to faintly translucent nodules and lens-shaped concretions with smooth, rounded, sometimes botryoidal surfaces. Colors run from liver-brown and chocolate to grey, tan, and bluish-grey, often with a banded or zoned interior.
- Color: brown, liver-brown, grey, beige, bluish-grey; concentric zoning common
- Luster: dull, waxy, to slightly resinous; never the fiery flash of precious opal
- Transparency: opaque to slightly translucent at thin edges
- Habit: nodules, flattened lenses, and tabular concretions in shale/marl
Step-by-Step Field-ID Checklist
- Look at the shape: menilite occurs as smooth nodules and lenses, often weathering out of dark marl or shale beds.
- Check for play-of-color: rotate in sunlight — menilite shows NO rainbow flash, confirming it is common opal, not precious opal.
- Test the conchoidal fracture: broken surfaces are smooth, curved, and glassy-to-waxy.
- Note the host rock: it is typically embedded in or weathered from organic-rich marl/shale (Menilite Formation type).
- Feel the weight and surface: lighter than agate of equal size; surface is waxy rather than glassy.
Key Diagnostic Tests
- Hardness: 5.5-6.5, softer than quartz/agate (7). A quartz point will scratch menilite; menilite will not scratch a steel file as readily as agate.
- Streak: white.
- Fracture: conchoidal, no cleavage.
- Acid: no reaction with dilute HCl (distinguishes from carbonate concretions, which fizz).
- Density: ~1.9-2.2, noticeably lighter than chalcedony (~2.6), a useful hand-feel clue.
- Water: being hydrous, some opal may craze or lose translucency if dried.
Common Look-Alikes and How to Tell Them Apart
- Flint/chert: also dark nodules in sedimentary rock, but they are microcrystalline quartz, harder (7) and denser (~2.6). Menilite is softer and lighter; a scratch and a heft test separate them.
- Precious opal: shows play-of-color; menilite never does.
- Carbonate (calcite/siderite) concretions: fizz in acid and are softer/cleave; menilite does not react to acid.
- Jasper: opaque silica too, but harder (7), denser, and usually brighter red/yellow; menilite is duller brown and softer.
Where It Is Typically Found
Menilite opal is named for Ménilmontant (Paris). Its classic occurrence is the Menilite Formation (Oligocene) of the Carpathian flysch belt — Poland, Ukraine, Slovakia, Czech Republic — where brown opal nodules weather from dark bituminous marls and shales. Similar liver-opal concretions occur in organic-rich sedimentary basins elsewhere.
Frequently asked questions
How can you tell if it is real menilite opal?
Real menilite is a dull brown-to-grey nodular common opal with a waxy luster, no play-of-color, a hardness of about 5.5-6.5, a conchoidal fracture, and it feels noticeably lighter than agate. It weathers out of dark marl or shale.
What does menilite opal look like?
It looks like smooth, rounded brown or grey nodules and flattened lenses with a dull waxy surface, often showing concentric banding inside, and no rainbow flash.
Menilite opal vs flint — how do I tell them apart?
Both occur as dark nodules in sedimentary rock, but flint is harder (Mohs 7) and denser (about 2.6), while menilite is softer (5.5-6.5) and lighter (about 2). A scratch test and hefting the two will separate them.
Does menilite opal show fire?
No. Menilite is a common opal and shows no play-of-color or fire. If a brown opal flashes rainbow colors, it is a precious opal, not menilite.
Menilite Opal identified by the community
Recent Menilite Opal specimens identified with Rock Identifier.
