Banded Onyx

Description

Banded Onyx is a translucent to opaque, fine-grained to microcrystalline variety of calcite, characterized by distinct, often parallel, bands of varying colors. These bands are typically wavy or concentric, reflecting the depositional environment. It is not a true onyx (which is a chalcedony, a form of quartz) but is named for its similar banding patterns. Its primary mineral component is calcite, making it relatively soft and reactive to acids.

How to Identify

Color

Highly variable, often white, cream, yellow, green, pink, brown, or red, typically in distinct bands. Impurities like iron oxides (red, yellow, brown), manganese oxides (dark brown, black), and chlorite (green) contribute to the coloration.

Luster

Vitreous (glassy) to waxy, especially on polished surfaces.

Texture

Fine-grained to microcrystalline, often with a sugary or granular appearance on broken surfaces. Polished surfaces are smooth.

Crystal Form

Typically massive, forming layered, banded, or stalactitic structures. Individual calcite crystals are usually too small to be seen without magnification.

Cleavage

Perfect rhombohedral cleavage (three directions not at 90 degrees), though often not apparent in fine-grained massive forms.

Geological Environment

Forms in karstic environments (caves) as speleothems (stalactites, stalagmites, flowstones) or in the vicinity of hot springs and geysers where calcium carbonate precipitates from supersaturated waters.

Key Facts

• Hardness: 3 (Mohs scale)
• Specific Gravity: 2.71 g/cm³
• Crystal System: Trigonal
• Color: Highly variable, often banded; white, cream, yellow, green, pink, brown, red
• Luster: Vitreous to waxy
• Transparency: Translucent to opaque
• Fracture: Conchoidal to uneven
• Cleavage: Perfect rhombohedral in three directions
• Composition: Calcium Carbonate (CaCO3)

Quick Check

• Color: Banded, various colors (white, cream, yellow, green, pink, brown, red)
• Luster: Vitreous to waxy
• Streak: White

Physical Characteristics

• Crystal Habit: Massive, stalactitic, stalagmitic, layered, banded, granular
• Cleavage Type: Perfect rhombohedral {1011}
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to waxy

Formation

Banded Onyx, or onyx marble, is a form of travertine or flowstone, which is a chemically precipitated sedimentary rock composed primarily of calcite (calcium carbonate, CaCO3). It forms in caves as stalagmites and stalactites, or in hot springs, where supersaturated calcium carbonate-rich waters deposit layers of calcite. The banding, which gives it its 'onyx' designation (though mineralogically distinct from true onyx, which is a cryptocrystalline quartz), results from variations in water flow, mineral impurities (e.g., iron oxides for reds/yellows, manganese oxides for browns/blacks), and organic matter content during deposition. Over geological time, these deposits can be subjected to diagenesis and low-grade metamorphism, leading to recrystallization and increased density.

Usage

Historically and currently used extensively as an ornamental stone for architectural applications (e.g., flooring, wall cladding, countertops, decorative objects), sculptures, and carvings. Its translucency makes it particularly desirable for backlit applications.

Age Distribution

Can form in various geological periods, from Precambrian to Cenozoic, depending on the specific cave or spring deposit formation.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Carbonates

Group

Calcite Group

Crystal System

Trigonal

Chemical Formula

CaCO3

Composition

Calcium Carbonate

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