Basalt -

When molten lava cools, it forms a heavy, dark rock that is can look like pillows stacked on top of each other.    

http://volcanoes.usgs.gov/Imgs/Jpg/Photoglossary/20011005-0039_DAS_large.jpg

Obsidian -

When lava cools really quickly, it looks like very smooth, black glass.

   http://volcanoes.usgs.gov/Imgs/Jpg/Photoglossary/obsidian.jpg

Scoria -

Sometimes when lava cools, it sometimes traps little pockets of air, creating a light colored rock that is very light to hold.

http://www.uoguelph.ca/~sadura/igref/ig8.html

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