Coffee Appreciation

Question for anyone but I have a feeling bomcup will have* the *answer.
Why are some beans shiny, shiny black (and almost greasy to touch) and some a matt, coarse brown.. I figure it has to do with the roast but how and why.. And is one bad one good or make no difference at all?

The picture shows three roasts with the darkest being found in Naples, located in the southern half of Italy. The medium roast is representative of coffee found in Florence and the Central Italian style, and the Northern Italian roast might be found in a typical espresso bar in Milan.

http://www.bestcafes.com.au/espresso-coffee-a-complex-and-fragile-beauty