Pronunciation: ob•SID•ee•un
Description*: Obsidian is volcanic glass, so fresh obsidian looks like a broken piece of black or brown glass.
Volcanic glass forms naturally when magma is cooled so rapidly that the atoms do not have time to connect with one another in regular lattice structures. As a result of its irregular structure, glass is unstable and eventually breaks down (devitrifies) to form a variety of minerals that are stable at Earth's surface temperature and pressure. This process is usually completed within ~20 million years.
Older obsidian may include round blebs of light-color minerals that are growing at the expense of the glass. These devitrifying specimens are sometimes called "snowflake" obsidian.
*Description from Cronin, V.S., 2001, Geology laboratory projects for group learning: Primis McGraw-Hill, 156 pp., ISBN 0-07-252348-4
Used by permission of the author.
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