Opal-A

X-Ray amorphous opal (hydrous silica), sometimes divided into two subtypes:
——— Opal-AG: (Amorphous-Gel) (contains closely packed amorphous silica spheres, which at times form a diffraction grating to create precious opal). Since the packing of the spheres is similar to the structure of a gel, the subscript G ("gel-like") has been added (Flörke et al, 1991; Graetsch, 1994).
——— Opal-AN: (Amorphous-Network (commonly found as hyalite). It still contains spheres, but much smaller (Lee et al., 2022) and lacks a play of colours.

In the study of Lee et al. (2022) opal-AN was shown to mainly comprise aggregates of differently-sized nanospheres (<5 nm) that form banded features. The opal-AG studied comprised close-packed silica nanospheres, ~400 nm in diameter, and these can be ordered into regular layers which can diffract light, causing the play of colour in most precious opal (Jones et al, 1964; Sanders, 1964 & 1968).