  Camp fire starter. 
Commonly used fire starter: You carve off some shavings of magnesium from the block using your hunting knife, then strike the flint with the back of the knife to ignite the shavings.  I removed the striker bar (see Cerium for a video of this bar being sparked with a knife).  This one was purchased from Walmart in April 2002.   
How pure is the magnesium you get at Walmart?  Analysis by x-ray fluorescence spectroscopy at the Center for Microanalysis of Materials, University of Illinois (partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439) gives the following composition: 
    97.24% Magnesium 
    1.21% Zinc 
    1.11% Manganese 
    0.44% Iron 
So, not too bad.  It's not reagent grade, but it's decent technical grade magnesium. 
Source: Walmart 
Contributor: Theodore Gray 
Acquired: 15 April, 2002 
Price: $4 
Size: 3" 
Purity: 97.2%
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