How to identify Onyx Marble

To correctly identify Onyx Marble (Calcite (variety of 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 banded in shades of white, cream, yellow, green, brown, and red. Impurities like iron oxides (red/brown), chlorite (green), or organic matter (darker shades) contribute to the coloration.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to vitreous (glassy) on fresh surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fine-grained to medium-grained crystalline, often with a sugary or granular appearance. Characterized by distinct parallel banding.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically massive, forming layered or banded deposits. Individual calcite crystals are usually anhedral (lacking well-formed faces) within the mass, but can sometimes show rhombohedral cleavage.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect rhombohedral cleavage (three directions not at 90 degrees), characteristic of calcite. This can be observed on broken surfaces.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in caves as speleothems (stalactites, stalagmites, flowstones) or at the mouths of hot or cold springs as travertine terraces. These environments are characterized by the precipitation of calcium carbonate from supersaturated groundwater or surface water.

Key Facts

Hardness

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

Specific Gravity

2.71 g/cm³

Crystal System

Trigonal

Color

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

Luster

Waxy to vitreous

Transparency

Translucent to opaque

Fracture

Conchoidal to uneven

Cleavage

Perfect rhombohedral (three directions at 74°55')

Composition

Calcium Carbonate (CaCO3)

Physical characteristics

• Crystal Habit: Massive, banded, layered, stalactitic, stalagmitic, botryoidal
• Cleavage Type: Perfect rhombohedral (1011)
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Waxy to vitreous

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