What makes the mafic rocks dark in colour? Calcium or iron?

Question

I know that mafic rocks contain high amounts of both calcium and iron, but I don't know which one results in the dark colur.

Answer 1

http://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=what%20makes%20mafic%20rocks%20dark

Probably the iron, but read many of the search results here to be sure.