How to identify Onyx Marble

To correctly identify Onyx Marble (Calcite (banded variety)), 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, typically in distinct bands.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy when polished, dull to earthy on unpolished surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fine-grained to medium-grained, often with a sugary or granular feel on broken surfaces. Smooth and cool to the touch when polished.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically massive, microcrystalline to cryptocrystalline, forming banded layers. Individual crystals are usually not visible to the naked eye.

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, fine-grained specimens.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in caves (as stalactites, stalagmites, flowstone), around hot or cold springs (as travertine), and in other environments where calcium-rich waters precipitate calcite.

Key Facts

Hardness

3 (Mohs scale)

Specific Gravity

2.71 g/cm³

Crystal System

Trigonal

Color

Highly variable, often banded in white, cream, yellow, green, brown, red

Luster

Vitreous to waxy, sometimes dull

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, microcrystalline to cryptocrystalline, forming banded layers (speleothems, travertine). Individual crystals are typically microscopic.
• Cleavage Type: Perfect rhombohedral, {1011}
• Fracture Type: Conchoidal to uneven, sometimes splintery
• Tenacity: Brittle
• Luster Type: Vitreous to waxy on fresh or polished surfaces; dull to earthy on unpolished, weathered surfaces.

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