Abstract: Taishun stone has been esteemed as "the fifth seal stone" in China, yet its mineralogical characteristics and genesis have been received relatively limited scholarly attention. The Landai Taishun stone, distinguished by its vibrant colouration, is highly prized and widely appreciated. However, the mineral components and formation mechanism of this variety remain to be comprehensively elucidated. This study employed a suite of analytical methods to investigate the gemmological and micromorphological characteristics of Landai Taishun stone samples, thereby revealing the conditions and mechanisms of the formation of the Landai within Taishun stone. Petrographic observation, X-ray powder diffraction, and Raman spectroscopy, identified the primary mineral components of Landai Taishun stone. The matrix predominantly consists of pyrophyllite with minor amounts of diaspore and kaolinite, while the Landai is composed of corundum. The pyrophyllite exhibits a mixture of 2M and 1Tc polymorphs, with the 2M type being predominant. The chemical composition of corundom in the Landai contains Ti and Fe, which are the main factors responsible for its blue colouration. The mineral component of Landai Taishun stone is consistent with that of Qingtian Landing stone. Based on the phase diagram of rock mineral assemblages and previous data analysis, the mineral component and the polymorphic properties of pyrophyllite suggest that the rock may have formed in a metamorphic environment at temperatures about 410-430 ℃, favorable for the dehydration and metamorphic transformation of diaspore into corundum. Detailed micromorphological and energy dispersive spectroscopic (EDS) analyses revealed that the Landai portion comprises newly-formed corundum, exhibiting a plate-like morphology with porous features. These features are resulted from the transformation process of orthorhombic diaspore to trigonal corundum, driven by crystal face inheritance and subsequent misalignment adjustments, providing direct evidence of the dehydration-driven transformation of diaspore to corundum.