The effect of the calcination temperature at 800 and 1000 °C on the stable formation of the tricalcium phosphate with rhombohedral structure in the system of the hexagonal crystal family was determined, while its precursor was made from limestone obtained from nature in the Lumajang district, Indonesia. The rhombohedral tricalcium phosphate samples were prepared by sol-gel routine, then examined by several tests, such as, Fourie transform infrared spectroscopy to study the deficiency of -OH, X-ray diffraction test to study the microstructure of the tricalcium phosphate with the rhombohedral structure in a hexagonal crystal system, differential scanning calorimetry and thermogravimetry tests to study thermal characteristics, scanning electron microscopy and energy dispersive spectroscopy to study the surface topography and to obtain the atomic ratio Ca/P ~1.5. Finally, the UV-vis test found the optical energy gap, E g , from ~5.34 to ~5.41 eV for the sample calcined at 800 °C and E g ~5.19 to ~5.21 eV at 1000 °C.