amorphous Mineraloids

Obsidian (Snowflake Obsidian)

SiO2 (55-75 wt%) + Al2O3, FeO, MgO, CaO, Na2O, K2O (rhyolitic volcanic glass; variable)

Crystal Structure

CDL Notation

 #! Species: Snowflake Obsidian
#! System: Amorphous
#! Habit: Black glass matrix with white cristobalite spherulites
amorphous[glassy]:{massive}[inclusion:spherulitic]

Crystal System amorphous

Point Group none

Crystal Forms (1)

{massive}

Quick Facts

Hardness

5.0

Specific Gravity

2.33

Refractive Index

1.48

Optical Character

Isotropic (ADR in stressed varieties)

Interactive Preview

Open this crystal in the CDL Playground to rotate, export, and explore in 3D.

Open in Playground

Physical Properties

Crystal Systemamorphous

Hardness (Mohs)5.0

Specific Gravity2.33

CleavageNone

FractureConchoidal (sharp edges; most diagnostic macroscopic feature)

LustreVitreous to sub-vitreous

Optical Properties

Refractive Index1.48

Optical CharacterIsotropic (ADR in stressed varieties)

PleochroismNone

Colours

Black (magnetite crystallites)Mahogany (red-brown hematite patches in black matrix)Snowflake (cristobalite spherulites in black matrix)Golden or silver sheen (oriented gas bubble layers)Rainbow/fire (thin-film interference in layered glass)

Localities

Mexico (Hidalgo, Jalisco, Michoacan; rainbow and snowflake varieties)USA (Glass Buttes Oregon; Obsidian Cliff Wyoming)Armenia (Satanakar, Geghasar; major prehistoric trade source)Iceland (Hekla volcano)Italy (Lipari, Sardinia)Ethiopia (Hareri region; East African Rift)New Zealand (Mayor Island)

Common Inclusions

Sub-microscopic magnetite/hematite crystallitesCristobalite spherulites (snowflake obsidian)Oriented gas bubble layers (sheen varieties)Flow bandingPhenocryst fragments (sanidine, plagioclase, pyroxene)

Known Treatments

Surface coatings (occasional; detected by acetone test)

Crystal Forms

massive

Diagnostic Features

Conchoidal fracture (primary); flow banding under magnification; RI 1.48-1.51; SG 2.33-2.50 (lower than most gem minerals); isotropic under polariscope

Notes

Not an IMA mineral species; natural volcanic glass (mineraloid) formed by rapid solidification of rhyolitic to rhyodacitic lava. IMA-approved: N/A. Flow banding and conchoidal fracture confirmed by Zook (1973) Journal of Gemmology (DOI 10.15506/jog.1973.13.6.220) [VERIFIED]. Van Nostrand's Scientific Encyclopedia chapter (DOI 10.1002/0471743984.vse5189) [VERIFIED]. RI and SG from Read 7th ed. (DOI 10.4324/9780080507224) [PARTIALLY_SUPPORTED]. Snowflake obsidian: cristobalite variety. Sheen: oriented gas bubble interference.