How to identify Onyx

To correctly identify Onyx (Banded Chalcedony (SiO2)), 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

   Typically black and white, or brown and white bands. Can also be solid black (often commercially called 'black onyx' but scientifically solid black chalcedony). Other colors like red, green, or blue can occur due to impurities or artificial dyeing.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to vitreous (glassy) when polished; dull to waxy on unpolished surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, microcrystalline, conchoidal fracture surfaces. Feels cool to the touch.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline, meaning individual crystals are too small to be seen without high magnification. Forms in botryoidal, mammillary, or stalactitic masses, often filling cavities.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in the cavities of volcanic rocks (e.g., amygdules in basalt, geodes), hydrothermal veins, and sometimes in sedimentary deposits where silica-rich waters have permeated and deposited chalcedony.

Key Facts

Hardness

6.5-7 on the Mohs scale

Specific Gravity

2.58-2.64 g/cm³

Crystal System

Trigonal (cryptocrystalline aggregates)

Color

Black, white, brown, red, green; often banded

Luster

Waxy to vitreous

Transparency

Translucent to opaque

Fracture

Conchoidal

Cleavage

None

Composition

Silicon dioxide (SiO2)

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates, typically massive, botryoidal, mammillary, or stalactitic, filling cavities. Individual crystals are microscopic.
• Cleavage Type: None
• Fracture Type: Conchoidal, producing smooth, shell-like breaks.
• Tenacity: Brittle
• Luster Type: Waxy to vitreous

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