Opal is a unique and captivating gemstone, chemically distinct from most other minerals. Real opal is classified as a mineraloid because it lacks the rigid, organized crystalline structure found in true minerals. Its composition is hydrated amorphous silica (silicon dioxide and water). The water content in natural opal typically ranges between 6% and 10% of its weight. This combination forms a non-crystalline, gel-like structure that solidifies over millions of years. This distinct formation process gives the stone its one-of-a-kind optical properties.
The Defining Phenomenon of Play-of-Color
The most distinctive feature of precious opal is the internal display of spectral colors known as the play-of-color. This optical effect results from the stone’s microscopic physical structure. Precious opal contains sub-microscopic spheres of silica, stacked in an orderly, three-dimensional lattice. When white light enters the stone, it diffracts around these tiny, uniformly sized spheres, splitting the light into its component colors, similar to a prism.
The size and spacing of the silica spheres determine which specific colors are visible. The play-of-color is highly dynamic, changing dramatically as the stone or the light source moves. Gemologists describe the intensity of the color flash using terms like “fire” or “brilliance.” The arrangement of the color patches is called the pattern, which varies widely. Common patterns include pinfire, which displays tiny, dense pinpoints of color. The broad flash pattern features large, sweeping areas of color. A highly prized effect is the rolling flash, where sheets of color appear to glide across the surface as the opal is rotated.
Variations in Opal Body Tone and Clarity
Real opal exhibits a wide spectrum of appearances based on its base material, or body tone. This is the underlying color independent of the flashing spectral effects. The body tone significantly influences how the play-of-color is perceived, as a darker background provides greater contrast. Gemologists use a scale to grade this tone, ranging from N1 (jet black) to N9 (white).
Black Opal
Black opal is characterized by a dark body tone, typically falling between N1 and N4 on the scale. This deep background maximizes the vibrancy of the play-of-color, making black opal the most sought-after variety. A small amount of matrix or host rock can be visible on the back of a natural black opal. This prevents its body color from being perfectly jet black.
White or Light Opal
White or light opal has a pale body tone, generally graded as N7 to N9, and is sometimes described as milky. While these opals display the characteristic color play, the light background makes the spectral colors less intense. They are the most common type of precious opal and are often prized for their subtle, delicate glow.
Crystal Opal
Crystal opal is defined by its transparency or semi-transparency, allowing light to pass through the stone. This clarity enhances the visibility of the internal color layers, making the play-of-color appear to float within the gem. Crystal opals can have body tones ranging from near-colorless to dark. A black crystal opal has both transparency and a dark base.
Boulder Opal
Boulder opal forms naturally attached to its host rock, which is typically brown ironstone. The opal material occurs as seams or layers within this matrix. The finished gem is cut to include the ironstone backing. This natural backing gives boulder opal a dark appearance, often rivaling the contrast of black opal.
Spotting the Difference: Natural vs. Synthetic and Treated Opal
Identifying a genuine, untreated opal requires careful examination, as the market contains various synthetics, imitations, and composite stones. Synthetic opals, such as Gilson or Kyocera opal, are grown in a laboratory but share a similar structure to natural opal. They often look “too perfect,” displaying a uniformity in pattern and color that is rare in nature.
Synthetic Opals
A telltale sign of a synthetic is the “lizard skin” effect, where the color blocks show a repetitive, scaly pattern. When viewed from the side, synthetic opals may show a columnar structure, with the color layers appearing in neat, stacked columns. In contrast, a real opal looks solid and more chaotic, exhibiting fluid, irregular patterns.
Composite Opals
Composite opals combine thin layers of real opal with other materials to enhance color and durability. A doublet consists of a thin slice of precious opal glued onto a dark backing, such as potch or ironstone. A triplet sandwiches the thin opal layer between a dark backing and a clear, domed cap made of glass or plastic.
To spot these composites, inspect the stone from the side for a perfectly straight, even line where the opal meets the backing material, which indicates adhesive. Triplets often have a perfectly uniform, high-gloss dome that can appear plastic or glass-like. A flawless, jet-black backing is also suspicious, as solid opals often retain natural imperfections or an irregular surface on the back.
Treated Opals
Treated opals, particularly those from Ethiopia, are chemically altered to darken their body tone and mimic black opal. If the stone is jet black with no visible matrix or inclusions under magnification, caution is advised. Shining a bright white light through the stone (transmitted light) may reveal a deep, cherry-red transparency in a smoke- or sugar-acid-treated stone. Natural dark opals typically show a yellow or grey transparency.