Aluminium oxide is better known as sapphire, a highly scratch-resistant material with a hardness of 1,800 Vickers that is most frequently used as a watch crystal. But Richard Mille uses it to construct the entire case of the RM 56-01 Tourbillon model, milling all the parts of the case from a single block of sapphire. As if that were not complicated enough (it takes a staggering 1,000 hours plus to make the case), the base plate, centre bridge and third wheel of the manual winding tourbillon movement for this limited edition of 5 pieces are also in sapphire crystal.

Source: Europa Star February - March 2013 Magazine Issue