Re: Why is diamond a mineral, since it forms from coal, which is a rock?

This is a very good question and to answer it I will first give you a 
dictonary definition.

Mineral- a naturally occuring substance with a characteristic chemical 
composition expressed by a chemical formula (which for coal and diamond are 
the same: Carbon); may occur as individual crystals (which diamonds are) or 
may be disseminated in some other mineral or rock;...

Rock- a consolidated or unconsolidated aggregate OF mineral grains 
consisting of one or  more mineral species and having some degree of 
chemical and mineralogic constancy.

So from this the difference between diamonds and coal really comes down to 
internal structure of the material. The atoms of carbon are much more 
closely packed that coal's atoms.

I hope this helps your class and teacher.



-geologyjoe