The mesmerizing appearance of an opal, with its internal flashes of spectral color, often leads people to question its scientific classification. Given its hardness and geological origin, it seems natural to place it alongside other well-known crystallized gemstones. Understanding the difference between a highly ordered structure and one that only appears ordered is the key to clarifying the opal’s unique position in the world of mineral science.
Defining Crystalline Structure
A substance is defined as a true crystal or crystalline solid by a specific arrangement of its internal components. This arrangement requires the atoms, ions, or molecules to be held together in a highly ordered, repeating three-dimensional pattern. This periodic structure, known as a crystal lattice, must extend over a long range within the material. The smallest repeating unit of this pattern is called the unit cell, which dictates the overall symmetry and properties of the solid.
The existence of a repeating internal structure is what distinguishes a mineral from other naturally occurring solids. A mineral is formally defined as a naturally occurring, inorganic solid with a defined chemical composition and a characteristic crystalline structure. The microscopic, long-range atomic pattern is the sole scientific requirement for a substance to be considered a true crystal. Materials like diamond and quartz meet this strict criterion because their atomic arrangements are perfectly symmetrical and infinitely repeating.
Opal’s Classification as a Mineraloid
Opal does not satisfy the strict requirements for a crystalline solid because it lacks the necessary long-range, repeating atomic order that defines a crystal lattice. Because of this structural deficiency, opal is classified as a mineraloid, which is a substance that shares many characteristics with minerals but fails one or more of the formal criteria.
Opal is essentially hydrated silica, meaning its chemical formula is silicon dioxide with an indefinite amount of water incorporated into the structure (SiO₂·nH₂O). This variable water content contributes to the stone’s lack of fixed internal organization. A mineraloid is a naturally occurring, inorganic solid that possesses an amorphous, or non-crystalline, internal structure. Opal fits this classification because its structure is disordered and non-repeating at the atomic level, similar to glass.
The Hidden Architecture of Opal
Although opal is classified as amorphous and not a true crystal, its internal architecture is far from random. The substance is composed of microscopic spheres of silica that have precipitated out of a silica-rich solution. These tiny spheres are the fundamental building blocks of the material.
The critical distinction between the two main types of opal lies in how these spheres are arranged. Common opal, sometimes called “potch,” consists of silica spheres that are jumbled together in a random, disorganized mass. This lack of order explains why common opal is usually opaque or milky and does not exhibit any vibrant optical effects.
Precious opal, the gem variety prized for its visual display, has a more structured arrangement, sometimes described as quasi-amorphous. The microscopic silica spheres in precious opal are uniform in size and stacked in a regular, close-packed, three-dimensional array. This internal arrangement creates a natural diffraction grating, which is the mechanism behind the stone’s famous optical property.
When light enters the precious opal, it is diffracted by the orderly stack of silica spheres and the voids between them. This process separates the white light into its constituent spectral colors, which then flash and shift as the stone is turned, a phenomenon known as “play-of-color”. The highly ordered stacking of these spheres, while not a true atomic crystal lattice, is what makes precious opal a unique and visually stunning mineraloid.