How to identify Opal

To correctly identify Opal (Opal), 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

   Extremely variable. Precious opal exhibits a 'play-of-color' with iridescent flashes of red, orange, yellow, green, blue, and violet against a body color that can be white, black, gray, blue, green, or brown. Common opal can be white, gray, blue, green, pink, brown, or colorless, without play-of-color.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to waxy or resinous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, often conchoidal fracture surfaces. Can be botryoidal, reniform, or stalactitic in habit.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous; does not form crystals. Occurs as botryoidal, reniform, stalactitic, or massive aggregates, or as vein fillings and replacements of organic material (e.g., wood, shells).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Opal is amorphous and lacks a defined crystal lattice to produce cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in sedimentary rocks (especially sandstone and shale), volcanic rocks (e.g., rhyolite, basalt), and hot spring deposits. Often occurs in veins, nodules, or as replacements of organic material. Requires silica-rich solutions and specific conditions for deposition and hardening.

Key Facts

Hardness

5.5 - 6.5 on the Mohs scale. Relatively soft, making it susceptible to scratching.

Specific Gravity

1.98 - 2.25. Varies with water content; generally lower than quartz.

Crystal System

Amorphous (lacks a crystal structure).

Color

Colorless, white, yellow, red, orange, green, blue, pink, brown, black. Precious opal exhibits a 'play-of-color'.

Luster

Vitreous to waxy or resinous.

Transparency

Transparent to opaque.

Fracture

Conchoidal (shell-like) to uneven.

Cleavage

None.

Composition

Hydrated amorphous silicon dioxide (SiO2·nH2O).

Physical characteristics

• Crystal Habit: Massive, botryoidal, reniform, stalactitic, nodular, vein fillings, or as pseudomorphs after wood or shells.
• Cleavage Type: None.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy or resinous.

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