How to identify Obsidian in Rhyolite

To correctly identify Obsidian in Rhyolite (Volcanic glass (Obsidian) within a fine-grained felsic volcanic rock (Rhyolite)), 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

   Obsidian: Typically black, dark brown, dark green, or rarely red/gray. Rhyolite: Light-colored, often pink, gray, white, or light brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Obsidian: Vitreous (glassy). Rhyolite: Dull to earthy, sometimes waxy if cryptocrystalline.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Obsidian: Amorphous, conchoidal fracture, smooth. Rhyolite: Aphanitic (fine-grained), sometimes porphyritic (with larger crystals in a fine matrix), often exhibits flow banding.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Obsidian: None (amorphous). Rhyolite: Microscopic crystals, sometimes phenocrysts of quartz, feldspar, or biotite.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Obsidian: None. Rhyolite: None (due to fine-grained nature).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic domes, lava flows, caldera rims, and explosive eruption deposits in areas of felsic volcanism.

Key Facts

Hardness

Obsidian: 5-5.5 (Mohs); Rhyolite: 6-7 (due to quartz and feldspar content)

Specific Gravity

Obsidian: 2.35-2.50; Rhyolite: 2.3-2.6

Crystal System

Obsidian: Amorphous; Rhyolite: Cryptocrystalline to microcrystalline (no macroscopic crystal system)

Color

Obsidian: Black, dark brown, dark green; Rhyolite: Pink, gray, white, light brown

Luster

Obsidian: Vitreous; Rhyolite: Dull to earthy

Transparency

Obsidian: Opaque to translucent in thin pieces; Rhyolite: Opaque

Fracture

Obsidian: Conchoidal; Rhyolite: Irregular to subconchoidal

Cleavage

Obsidian: None; Rhyolite: None

Composition

Both are felsic, primarily SiO2 (typically >69 wt%), with minor Al2O3, K2O, Na2O, FeO, MgO, CaO. Obsidian is essentially supercooled liquid of rhyolitic composition.

Physical characteristics

• Crystal Habit: Obsidian: Amorphous, massive; Rhyolite: Aphanitic to porphyritic, massive, often flow-banded
• Cleavage Type: None for both
• Fracture Type: Obsidian: Conchoidal; Rhyolite: Irregular to subconchoidal
• Tenacity: Obsidian: Brittle; Rhyolite: Brittle
• Luster Type: Obsidian: Vitreous; Rhyolite: Dull to earthy

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