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Obsidian ("awb-SID-ee-an")

		Obsidian was once lava from a volcano. The lava cooled so fast that it formed a glass, instead of crystals. So far, we haven't found obsidian just lying around loose in our neighborhood. However, we bought a 50-pound bag of "Black Obsidian" from a landscape supply store and put it in our garden. To learn more about obsidian, scroll farther down this page.

	How to recognize obsidian
	Other rocks that look like obsidian
	Special things to look for
	Where obsidian came from
	How obsidian formed
	Other names for obsidian
	Links to Websites about obsidian

How to recognize obsidian

	Obsidian is mostly black in color. Surfaces and cracks may be stained tan or brown by weathering.
	Obsidian looks glassy -- in fact, it's made of naturally formed glass, not crystals.
	You cannot scratch obsidian with a nail or knife. (See more about the scratch test.)

Other rocks that look like obsidian

Coal:

	Some coal is black and kind of glassy, like obsidian.
	However, coal draws a black line if you scrape it on the sidewalk. (Obsidian draws a white line, or no line at all.)

There are no volcanoes in Chicago, so the obsidian
we find in our area had to come from somewhere else.
Somebody must have shipped it here by truck or train.
However, so far we haven't found out where it came from.

How obsidian formed

Obsidian was once lava from a volcano. The lava
cooled so fast that it formed a glass, instead of crystals.

Other names for obsidian

We use the scientific name "obsidian" for this rock,
but it is also known by other names:

	We've bought 50-pound bags of obsidian gravel labeled "Black Obsidian."
	Many people call obsidian "volcanic glass."
	Obsidian sometimes is found as smooth, rounded lumps that some people call "Apache tears."

Here are some ways to classify obsidian (by grouping it with similar types of rocks):

	Obsidian is an igneous rock, because it formed from melted rock.
	Obsidian is an extrusive igneous rock, because it formed from lava "extruded" onto the surface of the Earth.

Links to Websites about obsidian

Here's an Encarta® Concise Encyclopedia article about obsidian:
< http://encarta.msn.com/find/Concise.asp?z=1&pg=2&ti=761555559 >

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This page was created on May 2, 2001, and it was last updated on July 27, 2002.