Diamantina is a pleasant enough colonial town, but is not in my opinion in the same class as Ouro Preto or even of a number of other non-inscribed colonial towns in Minas Gerais such as Tiradentes. It hovers between “worth a detour” and “worth a visit” if you have the time and want to clock up another WHS. The trouble is that it is somewhat isolated from the more frequented tourist areas south of Belo which can be passed through on a “natural route” and a “detour” to see it is quite long. If you decide to go there you will probably have to accept doing a return journey on public transport which could involve a couple of days. We avoided this by renting a car from Belo having bussed in overnight from Brasilia (if you do this go out to the closer and less busy Pamphula airport and “play the field” for a good deal with the range of rent-a-car companies there. While you are there visit the nearby Tentative List entry of “Ensemble architectonique de tourisme et loisir au bord du lac de Pampulha”) and were able to get up to Diamantina that morning AND back to Ouro Preto for that evening. A bus from Belo will take up to 6 hours each way (there are apparently ways of avoiding a return journey on public transport by changing at Curvelo near the Belo - Brasilia highway).
Diamantina’s relative isolation means that it is considerably less touristy than Ouro Preto etc. We however found it quite bustling with traffic (if anything more so than Ouro Preto). There is the usual selection of Baroque churches, colonial mansions and cobbled streets. The market square and its wooden building dating back to 1838 (photo) is perhaps its most unusual and “atmospheric” sight with the horses (belonging to the stallholders?) “parked” outside (though I had to work hard to avoid getting any cars in the picture!). It is said (though I have never seen “first hand” evidence) that this building with its archways was used by Niemeyer as inspiration for the Palacio da Alvorada in Brasilia (He was in Diamantina for the design of the Hotel da Tijuco). As you approach by road you get a feeling for the wildness of the countryside around Diamantina and, with more time, it might repay exploration – but we didn’t.