Opal, with its captivating play of colors, often leads to the question of whether it is a crystal. Despite its dazzling appearance and solid form, opal is fundamentally not a crystal. This distinction arises from its internal atomic arrangement, which differs significantly from the ordered, repeating patterns characteristic of true crystalline materials.
Understanding Crystalline Structures
A crystal is defined by an internal, highly ordered arrangement of atoms, ions, or molecules. This precise, repeating pattern, a crystal lattice, extends consistently in all three dimensions. Examples of true crystals include common substances like table salt, snowflakes, and minerals such as quartz. The external shape of a crystal often reflects this underlying atomic order, though the internal arrangement is the defining characteristic.
This inherent internal order allows crystals to exhibit predictable physical properties. The consistent spacing and bonding within the lattice determine how light interacts with the material, its hardness, and how it might cleave or fracture.
The Amorphous Nature of Opal
Opal’s internal composition stands in contrast to the ordered structure of crystals, categorizing it as an amorphous substance. Instead of a rigid, repeating atomic lattice, opal is primarily composed of microscopic spheres of silicon dioxide (silica). These tiny spheres are irregularly packed together, though in precious opal, they can arrange in more ordered, localized patterns.
Water is also an integral component of opal, typically making up between 3% to 21% of its weight. This water resides within the voids and spaces between the silica spheres. The presence of these spheres and the varying water content prevent the formation of a continuous, repeating crystalline structure.
In precious opal, the uniform size and regular stacking of these silica spheres create a natural diffraction grating. As light passes through this microstructure, it is diffracted and separated into its spectral colors, producing the characteristic “play-of-color” effect. Conversely, common opal, or “potch,” lacks this organized arrangement of spheres, resulting in no play-of-color.
Classifying Opal
Due to its lack of a defined crystalline structure, opal is scientifically classified as a mineraloid. A mineraloid is a naturally occurring, inorganic substance that shares many characteristics with minerals but does not meet all criteria, specifically the requirement of an ordered atomic arrangement. Other examples of mineraloids include obsidian and amber.
Opal is specifically known as an amorphous hydrated silica, reflecting its chemical composition of silicon dioxide with varying amounts of water. This classification distinguishes it from crystalline forms of silica, such as quartz, which possess a rigid, repeating crystal lattice and typically contain little to no water.