Carved in Stone

| Published on February 1, 2011

O is for obsidian

Although a relative youngster in geologic terms, obsidian can change over time from glass to mineral.

By Bill Langer

Obsidian (ŏb-sĭd’-ē-ən) noun – a usually black or banded, hard volcanic glass formed by rapid cooling of lava that displays shiny, curved surfaces when fractured.

‘Among the various kinds of glass, we may also reckon Obsian glass, a substance very similar to the stone which Obsius discovered in Æthiopia.’ Gaius Plinius Secundus (a.k.a. Pliny the Elder), c. 77 A.D., Naturalis Historia, Liber XXXVI, Capitulum LVXVII.


Nana Langer (she was my father’s stepmother and one of my favorite relatives) gave me an obsidian arrowhead when I was a young boy. It was my first exposure to minerals and artifacts, and I kept it in a little cotton-lined box locked up in the strong box Nana also gave me.

Okay, I know obsidian is not actually a mineral because it lacks the necessary crystalline structure to be truly classified as such. Technically, it is a glass, and sometimes it is classified along with mercury, opal, and amber as a mineraloid.

Obsidian usually contains more than 70 percent SiO2 (silicon dioxide), the stuff used to make window glass, which is also a mineraloid. Some obsidian is nearly colorless, but trace amounts of various elements commonly give it color. Iron and magnesium, for example, make it dark green to dark brown to black. Some obsidian contains gas bubbles that produce effects such as a golden sheen (sheen obsidian) or a pearly, rainbow-like sheen (rainbow obsidian). Some obsidian is banded, and some looks like it contains tiny snowflakes (snowflake obsidian).

It is rare to find obsidian older than about 20 million years. In geologic time, this makes obsidian very young compared with most rocks that make up the Earth’s continents. This is because over geologic time, obsidian gradually changes from glass to mineral through a process known as ‘devitrification.’ The disordered silica molecules within the obsidian slowly rearrange from disordered glass into an ordered crystal structure. For example, the ‘snowflakes’ in snowflake obsidian are cristobalite (a type of quartz) crystals that have formed through devitrification of the original obsidian.

When obsidian breaks, the absence of mineral crystal structure causes it to fracture along smooth, curved surfaces like the inside of a clam shell. These are called ‘conchoidal’ fractures (like the curve of a conch shell), and where they intersect, the edges of the obsidian can be sharper than a razor. I learned this the hard way when mishandling my prized arrowhead, much to the horror of Nana. My parents and I all reassured her that it was no big deal.

Prehistoric people living in places of recent (in geologic time) volcanic activity recognized the utilitarian value of obsidian. Obsidian from each volcanic source area has its own unique set of elements, and trace element analyses have shown that early man traded obsidian for many hundreds of miles. Those people put obsidian to many uses, especially projectile points, cutting implements, scrapers, and jewelry.

Obsidian was also used for sculptures. The opening of this article quotes Pliny the Elder (23-79 A.D.). I like to quote Pliny when I want to convey the impression that something is ancient. In his book, Pliny goes on to describe a Greek obsidian sculpture believed to date from the time of Homer (c. 850 B.C.). According to Pliny, that artifact proved ‘the use of this material [obsidian] is of more ancient date than is generally supposed.’

Pliny referring to something as ancient…

Now doesn’t that beat all!

Bill Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey and can be reached at 303-236-1249 or via e-mail at blanger@usgs.gov.

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