Opal is a unique mineral prized for its dynamic visual character, often displaying a spectrum of hues within a single stone. This hydrated form of silica, a compound of silicon and oxygen, does not possess a fixed, single color like many other gemstones. The stone’s amorphous structure allows it to appear in various forms, ranging from transparent to opaque.
The Distinction Between Body Color and Play-of-Color
The colors seen in precious opal are determined by two separate characteristics: the body color and the play-of-color. The body color, also known as the background or base color, is the fixed, underlying hue of the stone when viewed without movement. This color can range from white, gray, or black to lighter tones like yellow or clear.
The play-of-color is the spectacular, shifting optical effect that flashes across the opal’s surface as the stone is turned. This phenomenon appears as patches of brilliant spectral colors, such as red, green, and blue, that move and change when the viewing angle shifts. While the body color provides the backdrop, the play-of-color results from the stone’s internal structure interacting with light, creating the gem’s signature iridescence.
How Opal’s Internal Structure Creates Color
The dazzling play-of-color is created by the microscopic internal structure of precious opal. Unlike most gemstones, opal is not crystalline; it is composed of countless tiny, uniform spheres of silica gel. These spheres are arranged in a tight, three-dimensional geometric pattern, creating a natural structure known as a diffraction grating.
When white light enters the opal, it passes through this organized arrangement of silica spheres and the minute gaps between them. The ordered structure splits the white light into its component spectral colors, much like a prism. This process, called light diffraction, causes the light waves to interfere, scattering only specific wavelengths back to the observer’s eye. The size and spacing of the silica spheres determine which colors become visible.
Smaller spheres, measuring around 150 nanometers in diameter, diffract the shorter wavelengths of light, resulting in blues and greens, which are the most common colors seen in opals. Conversely, larger spheres, closer to 350 nanometers, diffract the longer wavelengths, producing the rarer oranges and reds. If the spheres are randomly sized or unorganized, the light is scattered without splitting into colors, resulting in a common opal that lacks the play-of-color effect.
Major Opal Color Classifications
Opals are categorized into major types based on their body color and the presence or absence of the play-of-color phenomenon.
Black Opal
Black Opal is defined by its dark body tone, ranging from jet black to dark gray. This dark background creates the most dramatic contrast, making the play-of-color appear exceptionally vibrant and intense.
White Opal
White Opal is one of the most common varieties, distinguished by its light body color, typically white, light gray, or creamy yellow. The play-of-color in white opal tends to be softer and more pastel compared to the dark-bodied varieties.
Fire Opal
Fire Opal is valued for its fiery body color, which can be vivid yellow, orange, or red. This type often lacks the play-of-color, or has only minimal flashes, and is prized for its warm, saturated base hue.
Crystal and Common Opal
Opals that are transparent or translucent are called Crystal Opal, allowing light to pass through and highlight the inner color layers. Common Opal, sometimes called potch, is the most prevalent form and does not display any play-of-color. This lack of color effect occurs because its internal silica spheres are not uniformly arranged, meaning it exists only in its body color, which can be blue, green, pink, or many other shades.