The quartet of xanthine (X), a purine base ubiquitously distributed in most human body tissues and fluids, has been for the first time fabricated and visualized, as the first alternative purine quartet besides the known guanine (G)-quartet. The X-quartet network is demonstrated to be the most stable phase on Au(111). Unlike guanine, the fabrication of the X-quartets is not dependent on the presence of metal atoms, which makes it the first metal-free purine quartet. Th, X-quartet holds great promise to potentially construct artificial new DNA quadruplexes for genetic regulation and antitumor therapy. Moreover, both the X-quartet itself and the quartet networks favor homochirality, suggesting homo-chiral xanthine oligomers and the networks may have been formed as the precursors of the pristine oligonucleotides on primitive Earth.