Radiant Opal Hues

Color

Gemstones are typically evaluated based on hue, saturation, and tone. Tone measures light absorption or refraction, hue indicates the stone’s darkness or paleness, and saturation measures the intensity of the primary color.

Play-of-Color

Due to their unique play-of-color, opals are categorized separately. Play-of-color describes the display of different colors within an opal, a phenomenon resulting from its silica sphere structure and trapped liquid. Unlike other gems, opals lack a crystalline lattice structure. When silica spheres are symmetrically arranged, light bends through them, splitting into various spectrum colors.

Opals with disorganized spheres are known as common opals and lack the vibrant play-of-color, making them less valuable than precious opals.

The play-of-color in opals varies based on the size of gaps between silica spheres; small gaps reveal greens and blues, while large gaps show reds, oranges, and yellows. Reds and oranges are rare and thus highly valuable, while blues and greens, though more common, remain beautiful. These colors can form patterns like peacock tail, Chinese writing, flagstone, jigsaw, and stripes, among others.

In considering play-of-color, one must also assess how evenly the color spreads across the stone’s surface, with areas lacking play-of-color regarded as body tone.

Body Tone

The body tone of an opal refers to the darkness or lightness of the stone’s background behind or around its play-of-color. Ranging from dark (black) to very light (transparent), body tones are graded from N1 (jet black) to N9 (transparent crystal).

Black opals (N1-N4) are the most valuable, followed by semi-black opals (N5-N6), and lastly, white opal crystals (N7-N9). The stone should be viewed face-up, disregarding play-of-color, when evaluating body tone.

Body Tone Color

Besides darkness or lightness, the body tone of an opal can have a dominant color. Black-bodied opals, known as “black opal,” are rare and highly sought after. Opals can also have body colors like green, blue, purple, white, orange, or colorless.

Fire opals, prized for their bright reds, oranges, and yellows set against a clear backdrop, are among the most valuable. Ethiopian opals, although colorless, exhibit excellent play-of-color. Peruvian or blue opals and honey (yellow) opals are other examples of opal types based on body color.

Opals from various locations exhibit distinct body colors. South Australia commonly produces white or milky opals, New South Wales is known for black opals, and Ethiopia and Kenya yield white and fire opals. Peruvian opals are typically pastel, while Brazilian opals have a crystal body with a vivid play-of-color.

Brightness

Brightness, or brilliance, measures how much light an opal reflects back to the observer. It includes surface brilliance and internal play-of-color. A dull play-of-color reduces an opal’s value, although exceptions like Boulder or matrix opals can still be prized if their play-of-color remains brilliant.

Transparency

Transparency impacts the value of light-toned opals, such as crystal or white opals. Greater transparency allows deeper visibility of play-of-color, enhancing value. Water opal, prized for its near-transparent, colorless appearance, embodies the highest value in this category.

Treatment

Although treatments to enhance opals exist, they generally decrease value. Methods like heat treatment, sugar solution, and sulphuric acid aim to deepen play-of-color. Other treatments use oils, plastics, resins, and waxes to fill gaps or address crazing. Smoking opals with materials to darken the surface is another technique. Untreated opals, preserving their natural color and durability, are typically more valuable.