  Catalytic converter honeycomb. 
Catalytic converters contain two ceramic honeycombs like this.  Each is about 2" thick (look at the 3D rotation video to see the sides of it) and contains some mixture of palladium, rhodium, or platinum particles coated on the surface of the ceramic.  These particles catalyze (hence the name) the burning of any residual unburned fuel coming out the engine exhaust pipe.  I have a Popular Science column that shows how to use one of these to burn propane gas without a flame. 
Source: Farm & Fleet 
Contributor: Theodore Gray 
Acquired: 8 February, 2009 
Text Updated: 8 February, 2009 
Price: $70 
Size: 5" 
Purity: <1%
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