The diversity of rock types has created the Da Dia reef geological heritage in Phu Hanh village, An Ninh Dong commune, Tuy An district. Da Dia reef stretches about 300m along the coast of Phu Hanh village, An Ninh Dong commune, is a famous tourist attraction and is recognized as a Special National Monument. This is a very unique exposure of Pliocene basalt rocks of the Dai Nga formation. Basalt columns of different colors, lengths and cross-sections (usually pentagonal, sometimes quadrilateral, hexagonal) are placed close together in different positions: vertical, inclined, horizontal or curved, looking very strange. The stone columns are split horizontally into layers, each layer continuously looking like stacks of plates stacked closely together. That is why this exposure is called Da Dia reef.

Geological characteristics of the heritage and Outstanding value of geological heritage.

Aesthetics: The unique structure and morphology of Da Dia reef and the types of basalt that make it up, together with the surrounding natural landscape, create aesthetic inspiration not only for scientific research but also for the creation of various art forms (painting, photography, sculpture, etc.).
Geological heritage types: type D (rock) and type B (geomorphology).