Examples - diamond vs coal

The allotropes of carbon are the different molecular configurations that pure carbon can take.

Diamond is one of the best known allotropes of carbon, whose hardness and high scattering of light make it useful for industrial applications, and jewelry.
Amorphous carbon is the name used for carbon that does not have any crystalline structure. [1] and coal is known as amorphous carbon.

Carbon structures for different allotropes are shown above. First one in the first row is diamond, and first one in the third row is amorphous carbon.

While the marvelous diamonds above worth thousands of dollars, the value of same amount of coal is respectively nothing. The pictures are quite self-explanatory to the reason why.
See the related article, Why do we believe?

[1] http://en.wikipedia.org/wiki/Allotropes_of_carbon