Menilite Opal
Hydrated silica (SiO2·nH2O)
An opaque grey-brown common opal forming nodules and concretions, historically called liver opal for its dull brownish color.
- Mohs hardness
- 5.5-6.5
- Color
- Grey, brown, to liver-brown, opaque
- Type
- mineral
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Overview
Menilite is an opaque, dull grey-to-brown variety of common opal, historically known as liver opal for its liver-brown coloring. It typically occurs as nodules, lenses, and concretions within sedimentary rock.
The name comes from Ménilmontant in Paris, France, where it was first described. As hydrated silica it shows no play-of-color; its interest is mineralogical and aesthetic rather than gemological.
Menilite often forms in flattened, sometimes layered concretions and is found in marl and shale sequences, where it can be associated with fossil-bearing strata.
Formation & geology
Menilite Opal forms by low-temperature precipitation of silica within fine-grained sedimentary rocks such as marls and shales. Dissolved silica, often derived from siliceous microorganisms like diatoms, concentrates and hardens into opaline concretions and nodules.
These bodies grow within the sediment, sometimes around organic nuclei, producing the characteristic flattened or rounded shapes.
Classic occurrences are in the Paris Basin of France and in the Carpathian flysch deposits of Central and Eastern Europe, where menilite shales are a recognized rock unit.
How to identify it
Identify Menilite by its opaque, dull grey to liver-brown color and its occurrence as nodules or flattened concretions, with a waxy to dull luster and white streak. Hardness is 5.5-6.5.
It shows no play-of-color and has a conchoidal fracture typical of opal.
Look-alikes include chert and flint (harder, around 7) and ironstone concretions (often magnetic or heavier). Menilite's lower hardness, opaline feel, and association with menilite shales help distinguish it.
Uses & significance
Menilite Opal has limited gem use because it lacks play-of-color, but it is collected as a mineral and occasionally cut as cabochons or used as an ornamental stone for its earthy banding.
Its main significance is geological: menilite shales are important marker beds and source rocks in European flysch sequences, and the nodules can preserve fossils.
In metaphysical circles it is sometimes promoted as a grounding stone, but this is not scientific. Its real value is to collectors and geologists.
Frequently asked questions
Why is Menilite called liver opal?
Its dull, opaque liver-brown color resembles liver, which earned it the traditional name liver opal.
Does Menilite show play-of-color?
No. It is a common opal with no internal ordering, so it displays only a steady grey-to-brown body color.
Where is Menilite found?
It is named for Ménilmontant in Paris and is common in the Paris Basin and Carpathian menilite shales of Central Europe.
How do I tell Menilite from flint?
Flint is harder (about 7) and rings when struck, while menilite is softer opaline silica with a duller, waxier feel.
Menilite Opal guides
In-depth guides for identifying, valuing, and understanding Menilite Opal.
Menilite Opal identified by the community
Real specimens identified with Rock Identifier.
