Opal is a unique mineraloid that stands apart from most gemstones due to its distinctive internal structure and captivating visual effect. Unlike crystalline minerals like quartz, opal is classified as an amorphous solid, meaning its atomic components lack the regular, repeating arrangement of a true crystal. This form of hardened silica gel is prized primarily for the dynamic display of color it can exhibit. The combination of its chemical makeup and physical structure gives opal a set of properties that demand special consideration for its handling and preservation.
Chemical Composition and Amorphous Structure
The fundamental chemical composition of opal is hydrated silicon dioxide, represented by the formula \(\text{SiO}_2 \cdot \text{nH}_2\text{O}\). This means that water molecules are incorporated into the structure, distinguishing it from non-hydrated silica minerals. The water content is variable, typically ranging from 3% to 21% by weight, though gem-quality opals usually stabilize between 6% and 10%.
The term “amorphous” defines opal as a mineraloid rather than a mineral because it lacks a defined, long-range crystal lattice. Opal is composed of microscopic spheres of silica that have condensed and solidified from a silica-rich solution. These tiny, non-crystalline spheres accumulate in layers, forming a solid structure that resembles a packed colloidal gel. The presence and stability of the water within the structure are linked to the formation process and the overall integrity of the material.
Defining Physical Characteristics
Opal’s physical characteristics reflect its soft and hydrated nature. Its scratch resistance, measured on the Mohs scale of hardness, typically falls between 5.5 and 6.5. This rating indicates that opal is softer than common quartz, making it susceptible to scratching from everyday dust and grit.
The density of opal is low compared to most gemstones, with a specific gravity generally ranging from 1.98 to 2.25. When subjected to stress, the material exhibits a conchoidal fracture, meaning it breaks with smooth, curved surfaces similar to glass. Opal’s transparency varies widely, from nearly opaque to fully transparent, and its surface luster can range from a glassy, vitreous sheen to a duller, waxy appearance.
The Phenomenon of Play-of-Color
The most celebrated property of precious opal is its “play-of-color,” an optical effect caused by the diffraction of white light within its internal structure. This phenomenon results from the highly organized, three-dimensional array of uniform silica spheres packed together within the material. For play-of-color to occur, these spheres must be nearly identical in size and arranged in a regular, close-packed lattice.
The size of the silica spheres, typically between 150 and 300 nanometers in diameter, determines the specific colors that are visible. When white light enters the opal, the spaces between these spheres act like a natural diffraction grating, separating the light into its constituent spectral colors. As the opal is tilted, the angle of the incoming light changes, causing the diffracted wavelengths to shift and creating the characteristic flashes of color.
If the silica spheres are randomly sized or too small, the diffraction grating effect does not occur. This material is known as common opal, or “potch,” which lacks the dynamic color display of precious opal. The uniformity and size of the internal silica structure are the most important factors determining an opal’s optical value.
Practical Implications of Opal’s Properties
The unique physical and chemical characteristics of opal necessitate specific care to maintain the stone’s beauty and integrity. The low Mohs hardness of 5.5 to 6.5 means opals are prone to scratching, especially when set in rings or worn during manual activities. Jewelers often place opals in protective settings, such as bezels, to shield them from impact and abrasion.
The water content is the source of opal’s most significant vulnerability, as loss of this internal moisture can cause “crazing.” Crazing is the formation of a network of tiny, irreversible cracks that spiderweb across the stone’s surface and interior. This dehydration is typically triggered by exposure to high heat, sudden temperature fluctuations, or excessively dry environments.
Opals should be protected from harsh chemicals, like household cleaners, and sudden thermal shocks. Cleaning is best accomplished with lukewarm, soapy water and a soft cloth; ultrasonic and steam cleaners must be avoided due to the risk of inducing crazing. Storing opals in a stable, slightly humid environment, away from direct sunlight or heat sources, helps maintain the essential moisture balance and reduces the risk of damage.