Schiller Obsidian

Description

Schiller Obsidian is a naturally occurring volcanic glass characterized by a distinctive iridescent sheen or 'schiller' when viewed under direct light. This optical phenomenon, often displaying rainbow colors, golden, or silver hues, differentiates it from common black obsidian. The base color is typically dark, ranging from black to dark brown or gray, and the iridescence becomes visible as the stone is rotated. It is an amorphous material, meaning it lacks a crystalline structure.

How to Identify

Color

Typically dark (black, dark brown, dark gray) with an iridescent sheen (rainbow, gold, silver) visible when light reflects off internal layers.

Luster

Vitreous (glassy) to sub-vitreous.

Texture

Smooth, conchoidal fracture surfaces. Non-porous.

Crystal Form

Amorphous; lacks crystal structure. May contain microscopic crystallites (microlites) or gas bubbles that cause the schiller effect.

Cleavage

None. Exhibits conchoidal fracture.

Geological Environment

Found in areas of recent felsic volcanic activity, typically associated with rhyolitic lava flows, domes, and ignimbrites. Occurs in the outer, rapidly cooled margins of these volcanic bodies.

Key Facts

• Hardness: 5 to 5.5 on the Mohs scale.
• Specific Gravity: 2.35 to 2.60 g/cm³.
• Crystal System: Amorphous (lacks a crystal system).
• Color: Black, dark brown, dark gray, with iridescent sheens (rainbow, gold, silver).
• Luster: Vitreous (glassy).
• Transparency: Translucent to opaque, thin edges may be transparent.
• Fracture: Conchoidal (shell-like, very sharp edges).
• Cleavage: None.
• Composition: Primarily silicon dioxide (SiO₂) with minor amounts of aluminum, sodium, potassium, iron, and other trace elements. Typically 70-75% SiO₂.

Quick Check

• Color: Dark (black, brown, gray) with iridescent flashes (rainbow, gold, silver).
• Luster: Vitreous (glassy).
• Streak: White (due to its amorphous nature and high silica content, it produces a white powder when scratched on a streak plate, though it's too hard to streak easily).

Physical Characteristics

• Crystal Habit: Amorphous mass, lacking external crystal form.
• Cleavage Type: None.
• Fracture Type: Conchoidal, producing extremely sharp edges.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy).

Formation

Schiller Obsidian, like all obsidian, forms from the rapid cooling of felsic lava (rich in silica, typically rhyolitic) with a low water content. The rapid quenching prevents crystal growth, resulting in an amorphous glass. The distinctive 'schiller' or iridescence (often seen as rainbow colors, golden, or silver sheens) is caused by the diffraction of light from microscopic inclusions of mineral crystals (e.g., magnetite, hematite, or feldspar) or gas bubbles aligned in parallel layers within the glass during flow. These layers are often sub-micrometer in thickness and act as a diffraction grating.

Usage

Primarily used as an ornamental stone, for lapidary work (cabochons, carvings), and in jewelry. Historically, obsidian was used for tools, weapons, and surgical blades due to its conchoidal fracture producing extremely sharp edges. Schiller obsidian is particularly prized for its aesthetic qualities.

Age Distribution

Typically Cenozoic, but can be found in any geological period with recent volcanic activity.

Where to Find

Finding Tips

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Scientific Classification

Mineral Class

Not a true mineral (amorphous), but classified as a mineraloid.

Group

Volcanic Glass

Crystal System

Amorphous

Chemical Formula

SiO₂ (with impurities)

Composition

Felsic volcanic glass, typically 70-75% SiO₂, with Al₂O₃, Fe₂O₃, MgO, CaO, Na₂O, K₂O, and H₂O.

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