Diamantina really is very different from Ouro Preto for example, and not just another mining town. It's smaller (the historical remains can only be seen in the center of the current city) and soberer. I found that it has some basic beauty.
There were quite a number of Brazilian daytrippers around, but I didn't see any foreign tourists. Maybe that's because it takes 5 hours to get here by bus from Belo Horizonte, not the center of the world itself. It's quite a lively town, with music played in the streets and a number of good restaurants. And (of course) the Diamond Museum. Here you can see some real diamonds (there's a very strong man with a gun guarding them, and selling the entrance tickets). Also, there are paintings that depict how the slaves had to do the digging, and how their masters lived.
In the town center there are a number of good sights. I liked the pure and simple Igreja do Rosario. And also the small houses along the street from there to another church, do Carmo. Not to be missed too is the very blue passageway at the Casa da Gloria.