Obsidian

Obsidian is a naiturally occurrin volcanic gless furmed as an extrusive igneous rock.

Obsidian
General
Category	Volcanic glass
Formula (repeatin unit)	70–75% SiO2, plus MgO, Fe3O4
Identification
Colour	Deep black
Fractur	Conchoidal
Mohs scale haurdness	5–6[1]
Skinkle	Vitreous
Speceefic gravity	~2.4[2]
Optical properties	Leamin
References	[3]

It is produced whan felsic lava extrudit frae a volcano cuils swith wi meenimum creestal growthe. Obsidian is commonly foond athin the mairgins o rhyolitic lava pirrs kent as obsidian flows, whaur the chemical composeetion (heich silica content) induces a heich viscosity an polymerisation degree o the lava. The inhibeetion o atomic diffusion throu this heich viscous an polymerised lava explicates the lack o creestal growthe. Obsidian is haird an brickle; it fracturs wi vera jaggie rands, that haed been uised in the past in cuttin an piercin tuils, an haes been uised experimentally as surgical scalpel blads.^[4]