While some laments the use of a solid rotor on the Grönefeld Principia, the 22-karat solid red gold monobloc rotor of the Principia is actually one of my favourite part of the movement.
It is extensively decorated by hand and features a frosted finishing which I personally find very attractive. Another pro of the rotor is it turns on a ceramic ball bearing which helps prevent wear and eliminates the need for lubrication.