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obsidian

obsidian

SRI Petrography ID: 32

Names

[de] Obsidian
[en] obsidian

Description

Glassy volcanic rock of acid to intermediate chemical composition. Usually dark coloured (black), sometimes also laminated (flow textures).
Obsidian is formed by the chilling of rhyolithic lava

If containing gas bubbles or water obsidian can also be grey.

In thin pieces obsidian is transparent.

Color

Dark coloured, mostly black, sometimes laminated, grey and brown

Grain Size

glassy

Texture

no texture, sometimes flow textures

Minerals

no macroscopic visible minerals, sometimes microcrystallites, which form slowly as the glass devitrifies

Occurences

Iceland, Italy (Eolian Islands), Melos, Lipari, Sardinia, Greek islands (Santorini, Aegina,..)

Comments

Obsidian is of high archaeological importance as it was widely used for the making of small tools. Its predictable fractures and sharp cutting edges made it ideal for blades and arrowheads.

Classification

Class

IGNEOUS ROCKS

Broader Terms

glassy igneous rocks