How to identify Boulder Opal

To correctly identify Boulder Opal (Opal in matrix (SiO2·nH2O)), 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 itself exhibits a full spectrum of play-of-color (iridescence) against a background of the host rock, which is typically dark brown to black ironstone. The play-of-color can include red, orange, yellow, green, blue, indigo, and violet.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy on the opal, dull to earthy on the ironstone matrix.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The opal portion is typically smooth and conchoidal when fractured, while the ironstone matrix can be granular or earthy.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (non-crystalline) for the opal, though it can fill existing fissures and cavities. The ironstone matrix will show the texture of its constituent minerals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None for opal; the ironstone matrix may exhibit parting planes depending on its composition.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in sedimentary basins, specifically within ironstone concretions or boulders in weathered profiles of Cretaceous sedimentary rocks. These environments are characterized by silica-rich groundwater percolating through porous host rocks.

Key Facts

Hardness

5.5-6.5 on the Mohs scale for opal; the ironstone matrix can vary but is generally harder, around 6-7.

Specific Gravity

2.0-2.25 for opal; the overall specific gravity of Boulder Opal will be higher due to the denser ironstone matrix, typically 2.5-3.0.

Crystal System

Amorphous (non-crystalline) for opal.

Color

Opal: All spectral colors (play-of-color); Matrix: Brown, dark grey, black.

Luster

Opal: Vitreous to waxy; Matrix: Dull to earthy.

Transparency

Opal: Transparent to opaque; Matrix: Opaque.

Fracture

Conchoidal for opal; uneven to splintery for matrix.

Cleavage

None for opal; none to poor for matrix.

Composition

Hydrated amorphous silica (SiO2·nH2O) for opal, with varying amounts of iron oxides, silicates, and other minerals for the ironstone matrix.

Physical characteristics

• Crystal Habit: Massive, botryoidal, reniform, stalactitic, or as thin seams and infillings within the host rock.
• Cleavage Type: None
• Fracture Type: Conchoidal for opal, uneven to splintery for matrix.
• Tenacity: Brittle
• Luster Type: Vitreous to waxy for opal, dull to earthy for matrix.

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