Luster, a fundamental property in gemology, describes how a gemstone’s surface reflects light. While opal is celebrated for its internal color flashes, this external reflection is a separate, defining characteristic. Opal is classified as a mineraloid rather than a true mineral due to its amorphous structure. Understanding this surface quality is important for appreciating the stone’s full physical nature.
Defining Luster in Gemology
Luster measures the quality and intensity of light reflected from a material’s surface. This intrinsic property is independent of the stone’s body color or any internal optical effects. Gemologists categorize this surface appearance into two broad types: metallic and non-metallic.
Most gemstones, including opal, exhibit non-metallic luster, which has several subclassifications. Common examples include vitreous (glassy), resinous, waxy, and dull or earthy. The degree of luster is influenced by the gemstone’s refractive index and the condition of its surface.
The Unique Luster of Opal
Opal’s luster is classified as non-metallic, typically ranging from vitreous to waxy or resinous. Precious opal, known for its vibrant colors, often exhibits a vitreous, glass-like polish. This surface quality results from the stone’s hydrated, amorphous structure, composed of microscopic spheres of silica and water.
The high water content and lack of a rigid crystalline lattice give opal a softer appearance compared to highly refractive crystalline gems like diamonds. High-quality precious opal generally has a brighter, more vitreous polish than common opal, which often displays a duller, waxy surface quality because it lacks the internal color effect.
Distinguishing Luster From Play-of-Color
Luster, the surface reflection, is frequently confused with opal’s signature optical phenomenon, play-of-color. Luster reflects light off the external surface, while play-of-color involves the internal diffraction and scattering of light from within the stone.
This internal light effect occurs because precious opal contains an orderly, three-dimensional arrangement of uniform-sized silica spheres. When white light enters, these stacked spheres act as a natural diffraction grating, splitting the light into spectral colors. This creates the brilliant, shifting flashes of color seen when the stone is turned.
Play-of-color depends entirely on the internal microstructure, whereas luster depends solely on the external surface’s interaction with light. Play-of-color should also not be confused with opalescence, which is a hazy or milky sheen found in common opal that does not exhibit spectral flashes.
Factors Influencing Opal’s Surface Appearance
The perceived luster of a finished opal depends highly on external factors and maintenance. Applying a high polish enhances its natural vitreous luster, providing a bright, smooth finish. However, because opal is relatively soft, its surface is susceptible to scratching and abrasion, which quickly diminishes the polish and results in a duller appearance.
The stone’s water content, ranging from 3% to 21% by weight, is another important consideration. Rapid dehydration, often caused by sudden temperature changes or very dry conditions, can lead to crazing. Crazing creates internal and external cracks that cloud the stone and severely reduce its surface luster.