Granite is what happens when silica rich lava does not erupt and instead crystalizes underground. It is mainly composed of feldspar, quartz and biotite. It is exposed through weathering and uplift. 

Some gemstones are granites like:

K2 "Jasper" - K2 "jasper" is really a Azurite in granitic rock from Pakistan. Raman analysis by Garry Du Toit at GIA in Bangkok confirmed that the blue spots consist of azurite, which occur in a matrix of sodic plagioclase, quartz, and muscovite. (Details at the GIA Pgs 55-56)

Unakite - is an altered or epidote granite with green epidote, quartz and pink feldspar. It's primary occurrence is in Unaka mountain range of North Carolina. 

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