Opal is often admired for its stunning optical effects, leading many to assume it is a true crystal like a diamond or quartz. Opal is not a crystal because it does not meet the strict structural requirements that define a crystalline solid. Instead, opal is a hydrated form of silica, chemically related to quartz but structurally distinct due to its water content and internal arrangement. This unique composition places it into a separate geological category.
Defining Crystalline Solids
A substance is classified as a true crystal when its constituent atoms, ions, or molecules are arranged in a highly ordered, repeating pattern. This arrangement is known as a crystal lattice, and the regularity of this structure extends throughout the entire material, a quality referred to as long-range order.
Long-range order gives crystalline solids specific physical properties, such as predictable cleavage planes and a sharp melting point. Minerals like quartz (pure silicon dioxide) and diamond (pure carbon) are examples of this highly organized atomic structure. If a substance lacks this repeating, three-dimensional geometric pattern, it cannot be designated as a crystal.
The Amorphous Structure of Opal
Opal fundamentally fails the test for long-range order because its internal structure is amorphous, meaning it lacks a defined, repeating atomic lattice. The chemical formula for opal is \(\text{SiO}_2 \cdot n\text{H}_2\text{O}\), indicating it is silicon dioxide with a variable amount of water incorporated into its structure. This water content, typically ranging from 3% to 21% by weight, contributes to its structural instability and non-crystalline nature.
Opal’s structure is a packing of microscopic silica spheres, ranging from about 150 to 300 nanometers in diameter. In common opal, these spheres are randomly aggregated. Precious opal, famous for its “play-of-color,” forms when these silica spheres are stacked in a regular, three-dimensional array.
This ordered stacking is the key to the opal’s visual appeal. Light passing through this structure is diffracted by the spaces between the uniformly sized spheres, splitting into spectral colors. The specific colors seen depend on the sphere size and the angle of observation, a phenomenon explained by diffraction.
Opal’s Classification as a Mineraloid
Because opal lacks the necessary crystalline structure, it is formally classified as a mineraloid. A mineraloid is a naturally occurring, inorganic substance that meets most, but not all, of the criteria for a true mineral, which requires a definite internal crystalline structure.
The variable water content is a secondary factor supporting the mineraloid classification, since true minerals must have a definite chemical composition. Opal’s water percentage is not fixed, contributing to its designation as a hydrated amorphous solid. This amorphous nature makes opal susceptible to dehydration, which can cause the material to crack or craze over time.