How to identify Banded Onyx

To correctly identify Banded Onyx (Calcite (a variety of onyx marble)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

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

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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)

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

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