What is obsidian?

Obsidian is a glassy stone formed by volcanic activities. The color is mainly black but sometimes gray, red, green and so on. Other glassy igneous rocks are pitchstone and pearlite.

obsidian(Oki)	obsidian(Shirataki)	Obsidian(Himeshima)	pitchstone(Houraiji-san)	pearlite(Koba)

obsidian photo

Obsidian are formed by rapid cooling of magma (from rhyoritic to dactic). The more the velocity of cooling is rapid, the more obsidian becomes glassy.

Because special conditions such as the nature of magma, rapid cooling and so on are necessary, obsidian occurs only in limited volcanic areas.

Obsidian is broken with conchoidal fracture as glass and with sharp edge. Stone tools were made by using this nature in ancient times.

Obsidian hardens without releasing water (H₂O) because of high viscosity, therefore, obsidian contains less than 2 % water. If obsidian is heated, bubbles are formed by water.

Hydration layer is formed on the surface. The square of the thickness of hydration layer is proportional to the elapse of years, which is applied to dating.

Obsidian is not perfect glass and contains crytalline impurities. Crystallite is the previous stage of crystallization and microlite is the microscopic crystal. Obsidian often has spherulite as is shown in the photo of Shirataki obsidian. The differences of forms of crystallites and microlites are used for identification obsidian sources.

Crystallites in obsidian
Kirigamine	Wadatouge	Wadatouge	Wadatouge	Kouzushima	Kajiya
Nagano	Nagano	Nagano	Nagano	Tokyo	Shizuoka

crystallite photo

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