How to identify Rough Opal

To correctly identify Rough Opal (Opal (Hydrated Amorphous Silica)), 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

   Highly variable. Common opal can be white, gray, blue, green, yellow, orange, red, brown, or black. Precious opal exhibits a 'play-of-color' with iridescent flashes of spectral hues against a body color that can be light (white, yellow, cream), dark (gray, black), or transparent (crystal opal).

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

   Often smooth to conchoidal fracture surfaces. Can be botryoidal (grape-like), reniform (kidney-shaped), stalactitic, or massive. May appear dull or earthy when still embedded in matrix.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous; does not form true crystals. Occurs as nodules, veins, seams, or replacements of organic material (e.g., fossilized wood or shells).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Opal is amorphous and lacks a crystalline structure that would allow for cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in sedimentary rocks, particularly in areas with volcanic ash beds or weathered basalt. It forms in arid or semi-arid regions where silica-rich groundwater can percolate through porous rocks and deposit in voids. Also found in hot spring deposits (geyserite) and as a product of weathering in some igneous rocks.

Key Facts

Hardness

5.5 - 6.5 on the Mohs scale. This makes it relatively soft compared to quartz.

Specific Gravity

1.98 - 2.25. This is lower than most common gemstones due to its water content.

Crystal System

Amorphous (lacks a defined crystal structure).

Color

Highly variable: colorless, white, gray, black, red, orange, yellow, green, blue, pink, brown. Precious opal exhibits a '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 and replacements.
• Cleavage Type: None (amorphous).
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy or resinous.

Explore Rough Opal