Perlite is natural glass that forms from volcanoes.

Chemical Formula: n/a
Class: Volcanic pyroclasitc rock
Hardness: soft
Coloration: dark blueish green, brown, grey or white with a waxy luster

How to identify it/Unique Characteristics or Properties: Perlite, like obsidian, is also natural glass that forms from volcanoes. Unlike obsidian, however, perlite has quite a bit of water in it, as the rock cooled too fast for the water to escape. Perlite continues to acquire water over time, becoming quite saturated at times. Eventually, perlite splits into small balls and fragment.

Where to find it in…

The Northwest: Anywhere obsidian can be found
Other: Hungary, New Mexico USA has good specimens.
Past Uses: Perlite is used in insulation once it has been expanded. It is also used as plant soil because it retains water so well.
How it was Made: Perlite forms from lava cooling quickly and retaining large amounts of water. The quick cooling makes volcanic glass, and the water in it makes perlite instead of obsidian. The picture below is of obsidian as it is being converted to perlite.
Interesting Facts: If you heat perlite up high enough and fast enough, the water will evaporate and the rock will puff up like popcorn (same process, actually, water vapor in it evaporating and the corn expanding as the vapor leaves) to over 40 times its original size.