How to identify Precious Opal

To correctly identify Precious Opal (Opal (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

   Exhibits 'play-of-color' – flashes of spectral colors (red, orange, yellow, green, blue, violet) that shift with angle of observation. Body color can be white, black, gray, blue, green, yellow, orange, or colorless.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy or resinous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, conchoidal fracture surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous; typically occurs as botryoidal, reniform, stalactitic, or massive aggregates, or as vein fillings and nodules. Does not form distinct crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in sedimentary rocks (especially sandstone and claystone), volcanic rocks (rhyolite, basalt), and as a precipitate from hot springs. Often fills veins, cavities, and replaces organic material (e.g., wood, shells).

Key Facts

Hardness

5.5-6.5 on Mohs scale.

Specific Gravity

1.98-2.25 (variable due to water content).

Crystal System

Amorphous (no crystal system).

Color

Variable body colors (white, black, gray, blue, green, yellow, orange, colorless) with characteristic 'play-of-color'.

Luster

Vitreous to waxy or resinous.

Transparency

Transparent to opaque.

Fracture

Conchoidal (shell-like).

Cleavage

None.

Composition

Hydrated amorphous silicon dioxide (SiO2·nH2O).

Physical characteristics

• Crystal Habit: Massive, botryoidal, reniform, stalactitic, nodular, or as vein fillings. Never forms euhedral crystals.
• Cleavage Type: None.
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy or resinous.

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