How to identify Opal

To correctly identify Opal (Hydrated amorphous silicon dioxide (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

   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 orange. 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 true 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, thus it does not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Opal forms in sedimentary and volcanic environments. In sedimentary settings, it typically occurs in arid or semi-arid regions where silica-rich groundwater percolates through porous rocks like sandstone, mudstone, or diatomaceous earth. It can fill fissures, cavities, or replace organic material. In volcanic settings, it forms in hot spring deposits (geyserite) or as amygdule fillings in volcanic rocks where silica-rich hydrothermal fluids are present. It is often found in association with limonite, sandstone, rhyolite, and basalt.

Key Facts

Hardness

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

Specific Gravity

1.98 - 2.25. Varies with water content; lower specific gravity indicates higher water content.

Crystal System

Amorphous (lacks a crystal system).

Color

Extremely varied, including white, black, gray, blue, green, yellow, orange, red, pink, and colorless. Precious opal exhibits 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: Amorphous; typically massive, botryoidal, reniform, stalactitic, or as vein fillings and replacements.
• Cleavage Type: None.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous, waxy, or resinous.

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