How to identify Rough Opal

To correctly identify Rough Opal (Hydrated Amorphous Silica (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

   Highly variable. Precious opal exhibits a 'play-of-color' with spectral hues (red, orange, yellow, green, blue, 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.

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 to slightly uneven, often botryoidal or reniform in habit. Can be found as nodules, seams, or infillings.

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 infillings in veins and cavities.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Exhibits a conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in sedimentary rocks, particularly those derived from volcanic ash (tuff) or sandstone, where silica-rich groundwater has percolated. Also occurs in veins and cavities in igneous rocks, and as fossil replacements (e.g., opalized wood, shells).

Key Facts

Hardness

5.5 - 6.5 on the Mohs scale

Specific Gravity

1.98 - 2.25 (variable due to water content)

Crystal System

Amorphous (no crystal system)

Color

Highly variable, including colorless, white, black, gray, red, orange, yellow, green, blue, pink, brown. Precious opal exhibits play-of-color.

Luster

Vitreous to waxy or resinous

Transparency

Transparent to opaque

Fracture

Conchoidal

Cleavage

None

Composition

Hydrated Amorphous Silica (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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