5.88 Ct. Cabochon Semi-Black Opal from Australia

This semi-black opal, supplied by The Natural Gemstone Company, is a 5.88 carat, opaque freeform specimen measuring 20.05 x 13.89 x 3.89 millimeters, exhibiting intense color intensity and an excellent polish, with no enhancements and Australian origin. The piece is presented in a freeform, non-faceted state, noted as cut n/a, which is the technically appropriate approach for opals that display strong internal color phenomena. The freeform profile preserves optimal thickness across the play-of-color zones, while the meticulous lapidary finishing has produced a high-gloss dome that maximizes light transmission into the stone. As a semi-black opal, its naturally darker body tone provides elevated contrast between the base color and the diffraction-generated flashes, allowing the chromatic bands to resolve with greater saturation and apparent brightness than in lighter opal varieties.

The unmatched sparkle of this opal arises directly from its internal microstructure, a quasi-periodic array of silica spheres whose diameters and packing order function as a three-dimensional diffraction grating. Incident light is diffracted by these ordered silica spherules, with sphere size distribution controlling the peak wavelengths produced, so smaller ordered domains reinforce blue and green, while larger domains produce orange and red. In this specimen the dominant sphere size distribution yields a multi-color palette with pronounced green and blue play, punctuated by spots of orange, indicating localized zones of slightly larger spherules. The semi-black body tone increases the modulation depth of the diffracted signal by attenuating background reflectance, which enhances contrast and makes each color flash appear more saturated and crisp. The excellent polish minimizes surface scattering and preserves specular entry of light into the diffractive layers, while the preserved thickness prevents destructive interference and loss of long-wavelength color. The freeform shaping further contributes to the dynamic visual performance by creating variable surface slopes that alter incident angles as the piece moves, producing a lively sequence of broad flashes and finer pinfire patterns that are visible across viewing angles. For jewelry fabrication we recommend settings that allow directional light access and, where appropriate, a darkened or open-back mounting to maintain the semi-black character, and we advise avoiding heat and ultrasonic exposure to preserve the native structure and play-of-color. For more technical details on this stone or to discuss precision mounting options, contact The Natural Gemstone Company.