Abstract: The opal crystalline state and colour patch have always been a hot issue in the gemmological community. However, very few literatures are focused on origins of the "silky" and "honeycomb-like cellular" colour patches, an important feature for the differentiation between natural and synthetic opals. For this reason, the authors tested and analyzed many opal samples using conventional gemmological test, SEM, and XRD. The research shows that the opal is in a continuous gradual changing state ranging from the noncrystalline to the crystallite to the sub-microscopic cryptocrystalline and finally to the cryptocrystalline. In addition, no distinct boundary is present between each two states. Furthermore, the opal may exist as an aggregate in all these different states. In this paper, the authors propose a gemological criterion to differentiate between the opal and the chalcedony, and also show different mechanisms for the formation of these two different colour patches. The "silky" colour patch occurs when the light is dispersed from the microscopic fissures inside the natural opal, but the "honeycomb-like cellular" colour patch is determined by the distribution of the SiO₂ spheres and the light transmittance characteristic of the spheres themselves within the synthetic opal. The understanding of the origins of these two colour patches has provided us with the theoretical basis for the differentiation between natural and synthetic opals.