amorphous Mineraloids

Obsidian

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

Crystal Structure

CDL Notation

 #! Species: Obsidian (volcanic glass)
#! System: Amorphous (mineraloid)
#! Habit: Massive with conchoidal fracture; flow structures
amorphous[glassy]:{massive}

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)

Physical Properties

Crystal System: amorphous
Hardness (Mohs): 5.0
Specific Gravity: 2.33
Cleavage: None
Fracture: Conchoidal (sharp edges; most diagnostic macroscopic feature)
Lustre: Vitreous to sub-vitreous

Optical Properties

Refractive Index: 1.48
Optical Character: Isotropic (ADR in stressed varieties)
Pleochroism: None

Characteristics

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.