Obsidian – Black Glass Sharper than Diamond

Today’s featured rock/mineral piece is on: Obsidian.


Obsidian is a volcanic glass created when magma is extruded from a volcano and quickly becomes quenched (cooled) so that crystals have almost no time to grow. The result is a silicon dioxide glass with magnesium and iron to give it the dark color.

Obsidian hand sample.
Obsidian hand sample showing the glassy texture.
Surprising Things about Obsidian

Obsidian is considered more of an igneous rock than a mineral, since glass does not have the crystal structure that defines minerals.

Obsidian has a conchoidal (shell-like pattern) fracture, which means that it breaks like glass and does not form any flat crystal facets. Because of its sharp edges early man used it to make cutting tools or arrowheads.

Surprisingly, the edge of a piece of obsidian is superior to that of a surgeon’s steel scalpel. It is 3 times sharper than diamond and between 500-1000 times sharper than a razor or a surgeon’s steel blade resulting in easier incisions and fewer microscopic ragged tissue cuts. Given these unique characteristics a German company has created surgical tools that allow doctors to use obsidian instruments on patients who might be allergic to steel. Another benefit to using obsidian scalpels is the reduction of healing time for surgical incisions.

Obsidian under the microscope showing microcrystalline texture.
Obsidian under the microscope showing a complex fine-grained texture. The fine-grain is due to quick cooling magma.

Obsidian is found all over the world. In the U.S. it is found in volcanic areas or areas were volcanism has occurred sometime in Earth’s history.


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