Granite is formed out of at least three different minerals:

---always QUARTZ

---always FELDSPAR (usually ORTHOCLASE)

---always a dark mineral (usually BIOTITE MICA and/or HORNBLENDE,)

 

It can have more than one kind of feldspar in it, or have other minerals, too.  But it has to have QUARTZ, FELDSPAR,  and dark minerals like BIOTITE MICA or HORNBLENDE.  Usually it is light colored.  You can also see the mineral grains just by looking carefully.

It also has to have a certain amount of each of them.  The recipe for GRANITE is:

27% or more Quartz

3-10% dark mineral

63-70% Feldspar and other minerals

 

For more information try these links:

http://stoneplus.cst.cmich.edu/granite.htm

http://uts.cc.utexas.edu/~rmr/definition.html

http://geology.com/rocks/granite.shtml

http://geology.about.com/od/more_igrocks/a/granite.htm