Development of sustainable synthesis of La₂O₃ nanocrystals formation employing Datura metel leaf extract was explored. The obtained nanocrystals were analyzed employing XRD, Raman, PL, FTIR, XPS and TEM characterizations. With increasing incubation and aging, the transformation of La(OH)₃ to LaOOH and metal ellagate complex formation to La₂O₃ nanocrystals formation was observed. The obtained XRD results clearly revealed the transformation of lanthanum hydroxide to lanthanum oxide hydroxide and then to lanthanum oxide nanocrystals formation with 1, 4 and days. The influence of incubation and aging on La₂O₃ nanocrystals formation was discussed. Datura metel leaf extract product mixture over a period of incubation formed pure hexagonal lanthanum oxide nanocrystals.