We report here the unprecedented preparation of Reuleaux triangle disks. The hydrolysis and precipitation of bismuth nitrate in an ethanol-water system with 2,3-bis(2-pyridyl)pyrazine yielded basic bismuth nitrate Reuleaux triangle disks. Analysis of the intermediates provided insights into the mystery behind the formation of the Reuleaux triangle disk, revealing a unique growth process. The report of a facile method to prepare crystals of a novel shape in high yield, with good homogeneity, and with excellent reproducibility is expected to unlock new research directions in multiple disciplines.