Moss Opal

What is moss opal?

Moss opal is a translucent to semi-transparent variety of common opal characterized by green, brown, or black dendritic inclusions that create patterns resembling moss, ferns, or algae. These inclusions consist primarily of manganese or iron oxide minerals that formed during the opal’s crystallization process.

The stone forms in volcanic regions where silica-rich solutions penetrate rock cavities, allowing the simultaneous precipitation of opal and metallic oxides. Found predominantly in Montana, USA, and select locations in Kazakhstan and India, moss opal exhibits a hardness of 5.5-6.5 on the Mohs scale. The stone’s unique optical properties include a vitreous to resinous luster, with some specimens displaying weak play-of-color effects beneath their surface patterns.

Unlike precious opal, moss opal lacks a structured arrangement of silica spheres, resulting in its classification as a mineraloid rather than a true mineral. The stone’s distinctive appearance makes it valuable in both lapidary work and geological studies of hydrothermally altered volcanic environments.

Type of Opal (Natural vs Synthetic)

Moss opal is typically a natural gemstone.

Synonyms / Common Names

Common names for Moss opal include Dendritic opal, Merlinite, and Tree agate.

Hardness:

Moss opal, a variety of common opal, typically has a hardness of 5.5 to 6.5 on the Mohs scale. This makes it relatively soft compared to many other gemstones, meaning it can be more susceptible to scratches and abrasion.

Refractive index (ri):

The refractive index of moss opal ranges from approximately 1.44 to 1.46, which is generally lower than many crystalline gems. This characteristic contributes to its subtle optical effects compared to more brilliant gemstones.

Luster:

Moss opal exhibits a waxy to dull luster, which is typical for opals that do not display the play-of-color seen in precious opals.

Cleavage:

Moss opal, like other forms of opal, has no cleavage. This means it does not break along well-defined planes of weakness, which is a typical characteristic of amorphous materials like opal.

Fracture:

The fracture of moss opal is typically conchoidal (shell-like), which is common among amorphous materials.

Specific gravity (sg):

The specific gravity of moss opal tends to range from about 2.1 to 2.3, which is relatively low compared to many other gemstones. This low density can make larger specimens feel surprisingly light.

Optical birefringence (double refraction):

Moss opal does not exhibit double refraction, as it is an amorphous solid rather than a crystalline material.

Dispersion (fire):

Moss opal generally displays minimal to no dispersion or fire, which is the splitting of white light into its constituent spectral colors.

Crystal system:

Being an amorphous material, moss opal does not have a crystal system.

Color:

The color of moss opal includes a translucent to opaque base typically in white, grey, or brown, with inclusions that resemble moss, fern, or other plant-like patterns. These inclusions are usually green, black, or brown.

Transparency:

Moss opal ranges from translucent to opaque, depending on the concentration and type of impurities or inclusions within the stone.

Pleochroism:

Due to its amorphous nature, moss opal does not exhibit pleochroism.

Fluorescence:

Moss opal typically does not fluoresce under UV light, although there can be exceptions depending on specific impurities or inclusions.

Toughness:

Moss opal is generally considered to have fair to good toughness, which makes it reasonably durable for wear in jewelry that is not exposed to rough conditions.

Tenacity:

The tenacity of moss opal is brittle, which is typical for opal.

Optic sign:

As an amorphous gem, moss opal has an isotropic optic sign.

Absorption spectrum:

Moss opal does not typically show a distinctive absorption spectrum due to its lack of crystalline structure and uniform color distribution.

Chemical composition:

Moss opal consists primarily of silica (SiO2) with a variable water content. It may also include other trace elements that contribute to its color and moss-like inclusions.

Chatoyancy (cat‚äôs eye effect):

Moss opal does not generally display chatoyancy, as this effect is more commonly associated with fibrous or needle-like inclusions found in other types of gemstones.