How to identify Boulder Opal

To correctly identify Boulder Opal (Opal in matrix), 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

   Opal exhibits a full spectrum of colors (red, orange, yellow, green, blue, violet) in its play-of-color. The ironstone matrix is typically dark brown, reddish-brown, or grey. The overall appearance is a vibrant opal color against a dark, earthy background.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Opal: Vitreous to waxy. Ironstone matrix: Dull to earthy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Opal: Smooth, conchoidal fracture surfaces. Ironstone matrix: Granular, often rough or pitted.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Opal is amorphous, lacking a defined crystal structure. It forms botryoidal, reniform, or stalactitic masses, or as thin seams and infillings. The ironstone matrix is typically massive or concretionary.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Opal: None. Ironstone matrix: None.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Boulder opal is found in sedimentary environments, specifically within the weathered profiles of Cretaceous-age ironstone formations. It occurs in veins, seams, or nodules within the ironstone, often in association with kaolinite and other clay minerals.

Key Facts

Hardness

Opal: 5.5-6.5 on the Mohs scale. Ironstone matrix: Varies, typically 4-7 depending on composition (e.g., goethite, hematite, quartz).

Specific Gravity

Opal: 1.98-2.25. Boulder Opal (with matrix): Varies significantly depending on the proportion of opal to ironstone, typically higher than pure opal.

Crystal System

Amorphous (Opal). Ironstone matrix is typically massive or microcrystalline.

Color

Opal: Colorless, white, grey, black, red, orange, yellow, green, blue, pink, purple; often with play-of-color. Matrix: Dark brown, reddish-brown, grey.

Luster

Opal: Vitreous to waxy, pearly, or resinous. Matrix: Dull to earthy.

Transparency

Opal: Transparent to opaque. Matrix: Opaque.

Fracture

Opal: Conchoidal. Matrix: Uneven to hackly.

Cleavage

None (Opal and Ironstone).

Composition

Hydrated amorphous silica (SiO2·nH2O) for opal, primarily iron oxides (e.g., goethite, hematite) and quartz for the ironstone matrix.

Physical characteristics

• Crystal Habit: Opal: Massive, botryoidal, reniform, stalactitic, or as thin seams and infillings. Matrix: Massive, concretionary.
• Cleavage Type: None.
• Fracture Type: Opal: Conchoidal. Matrix: Uneven to hackly.
• Tenacity: Brittle.
• Luster Type: Opal: Vitreous to waxy. Matrix: Dull to earthy.

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