Each of the five Ultrathin Tourbillon Dragon & Phoenix models features a dial made from a different hardstone. Bronzite, Eudialyte, Marcassite, Pietersite and Verdite are coloured minerals with deep surface effects, but are little known and rarely used in watchmaking. Arnold & Son chose these varieties of hardstone for their evocative properties. Black and grey Marcassite is scattered with outgrowths like stones emerging from a flat landscape.