How to identify Onyx Marble

To correctly identify Onyx Marble (Banded Calcite (Travertine)), 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, brown, red, or a combination, arranged in distinct parallel bands.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy, often duller on unpolished surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically massive, banded, stalactitic, or stalagmitic. Individual calcite crystals are usually microscopic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect rhombohedral cleavage (three directions not at 90 degrees), though rarely observed in massive forms.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in karstic environments, caves (as stalactites, stalagmites, flowstone), and around hot or cold springs where calcium-rich waters degas carbon dioxide, leading to the precipitation of calcium carbonate.

Key Facts

Hardness

3 on Mohs scale (can be scratched by a copper coin)

Specific Gravity

2.71 g/cm³

Crystal System

Trigonal

Color

Varied, often banded (white, cream, yellow, green, brown, red)

Luster

Vitreous to waxy

Transparency

Translucent to opaque

Fracture

Conchoidal to uneven

Cleavage

Perfect rhombohedral (3 directions at 74° 55')

Composition

Calcium Carbonate (CaCO₃)

Physical characteristics

• Crystal Habit: Massive, banded, stalactitic, stalagmitic, botryoidal, reniform
• Cleavage Type: Rhombohedral, perfect on {1011}
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to waxy

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