The second hardest material known to man after diamond, sapphire is almost impossible to scratch – making it a technical headache for watchmakers to turn the entirely transparent material into a one-of-a-kind timepiece. And yet that is exactly what Bell & Ross has achieved with its special-edition 45mm BR-X1, exclusive to Mr Porter. The exposed Swiss-made calibre BR-CAL.288 movement is encased in anti-reflective sapphire glass, showcasing the skeletonised blue dial with its Super-LumiNova-filled hour and minute indicators, and a  flying tourbillon that counters the effects of gravity on the intricate watch mechanism. Pity the courier tasked with delivering this one-off piece: it retails at £355,000.