Opal, a unique mineraloid, captivates with its remarkable display of color, unlike many gemstones that exhibit a singular hue. This distinctive quality stems from its inherent structure and the way it interacts with light. Its physical properties reveal a spectrum of appearances, from subtle background tones to vivid, shifting optical phenomena.
The Basic Nature of Opal’s Color
Opals present a wide array of colors. An opal’s appearance is primarily defined by two elements: its “body color” and its “play-of-color.” Body color refers to the underlying background hue, which can vary from white, black, or gray to clear, yellow, orange, or blue. Play-of-color is a dynamic display of spectral colors that shift within the stone as it moves. This internal iridescence distinguishes precious opals from common opals, which lack this phenomenon.
The Phenomenon of Play-of-Color
The shifting colors in precious opals result from light diffraction. Opal is composed of microscopic silica spheres arranged in a precise, ordered, three-dimensional pattern. These spheres, along with tiny gaps containing a water and silica solution, act like a natural diffraction grating.
When white light enters the opal, it passes through these uniformly sized spheres and the spaces between them. The light then diffracts, or splits, into its component spectral colors, similar to how a prism disperses light. The size and uniform arrangement of these silica spheres determine which colors are visible. Larger spheres, approximately 350 to 440 nanometers in diameter, typically produce warmer colors like red and orange. Smaller spheres, around 150 to 200 nanometers, tend to diffract cooler colors such as blue and green. Red in an opal is rarer, as it requires larger silica spheres.
Types of Opal by Dominant Color and Appearance
Opals are categorized based on their dominant body color and how their play-of-color manifests.
Black opals are highly prized for their dark body tone, ranging from dark gray to black, which provides strong contrast, making their vibrant play-of-color appear exceptionally bright and intense. Most high-quality black opals originate from Lightning Ridge, Australia.
White opals possess a light body tone, typically translucent to opaque white, often showcasing delicate pastel flashes of color. These are the most common type of precious opal.
Crystal opals are characterized by their transparent to translucent body, allowing light to pass through, often resulting in a vivid and clear display of play-of-color throughout the stone.
Fire opals are distinguished by their warm body colors, usually yellow, orange, or red. While they may exhibit play-of-color, it is not always present, and their appeal often lies in their fiery background hue.
Boulder opals feature a thin vein of precious opal naturally attached to its host rock, often ironstone, which serves as a dark, natural backing that enhances the opal’s colors.
Matrix opals have precious opal interspersed within their host rock, creating a mosaic-like pattern where the opal flashes appear embedded within the surrounding material.
Factors Influencing Opal’s Color and Value
Several factors contribute to an opal’s perceived quality and value. The brightness or intensity of the play-of-color is a significant determinant, with more vivid and brilliant flashes commanding higher value. Opals that display a full spectrum of colors are rarer and generally more valuable, particularly if red hues are present. The pattern of the color flashes also influences value; patterns like “harlequin” (broad, angular patches) or “pinfire” (small, closely set dots) are highly sought after. The body tone, or background color, plays a role, as darker body tones typically enhance the vibrancy of the play-of-color due to increased contrast, leading to higher valuations.