Opal

Although it is still (2007) regarded as a mineral species for historical reasons, opal is not a true mineral in the accepted sense of the word as it is variably composed of poorly crystalline material related to cristobalite and/or tridymite and/or composed of X-Ray amorphous silica. Commonly considered a mineraloid due to its amorphous nature, but more like a mixture due to its common but variable paracrystalline components.

Opal is structurally classified into three main types, though they grade into one another (Jones & Segnit, 1971; Curtis et al, 2019):
- Opal-CT: originally considered to contain cristobalite and tridymite but probably contains mostly poorly crystalline tridymite plus amorphous silica and sometimes poorly crystalline cristobalite
- Opal-C: contains poorly crystalline cristobalite-like material
- Opal-A: X-Ray amorphous, 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.

Transitions between opal-A, opal-CT, and opal-C are common, eg Curtis et al. (2019).

Studies at low temperature show that water molecules may be organized into an ice-like structure, which includes the cubic ice modification (Eckert et al., 2015).

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).

Opal with a play of colours (“precious opal”) is most commonly opal-AG but some is opal-CT ( Curtis et al, 2019). The diffracting structure and the resulting opalescence may be preserved to some degree when opal-AG transforms to opal-CT by crystallization (Sanders, 1975).