Moss Opal

Moss opal is a variety of common opal that contains dark, branching inclusions resembling moss, ferns or tiny shrubs trapped within a pale, translucent silica body. The plant-like patterns are not organic but are mineral dendrites, typically iron or manganese oxides, that grew along fractures and surfaces as the opal formed.

It is closely related to moss agate and dendritic opal, differing mainly in that the host is opal rather than chalcedony. Each stone is unique, with the "scenery" of inclusions giving it a natural, landscape-like appeal.

Valued for its decorative patterns rather than fire, moss opal is a favorite among collectors of picture stones and natural pattern gems.

Moss opal forms when hydrated silica precipitates from silica-rich water into cavities and seams in host rock, as with all common opal. The distinctive moss patterns develop when mineral-bearing solutions, usually carrying iron or manganese, infiltrate tiny cracks and surfaces in the silica.

As these solutions react and the metals oxidize, they crystallize into branching, tree-like dendrites suspended within or along the opal. The result depends on the chemistry and the geometry of the fractures, producing green (iron-rich), black or brown (manganese-rich) mossy inclusions. Because the silica spheres are irregularly arranged, moss opal generally shows no play-of-color, remaining a patterned common opal.

Identify moss opal by its translucent to whitish body crowded with green, black or brown branching, plant-like inclusions, with a waxy luster and moderate hardness (5.5-6.5). It shows no rainbow play-of-color; the interest is the dendritic pattern.

The closest look-alike is moss agate, which has the same dendrites but a chalcedony host that is harder (about 7) and typically more translucent and glassy. Testing relative hardness helps: opal is softer and lighter. Dendritic patterns are mineral, not fossil plants, so they appear two-dimensional along planes rather than as three-dimensional organisms. A white streak and conchoidal fracture confirm an opal host.

Moss opal is cut as cabochons, slabs and beads for jewelry and as decorative display pieces, prized for its unique natural "landscape" patterns. Because each piece is one of a kind, it appeals to designers and collectors of pattern and picture stones.

Metaphysically it shares opal's creative associations and moss agate's reputation as a grounding "stone of nature" connected to growth, abundance and the earth. As a softer, sometimes porous stone it should be protected from scratches, sharp impacts, heat and chemicals, and cleaned only with a soft damp cloth to preserve both the opal and its delicate inclusions.