Flame opal is a descriptive trade term for opal in which the play-of-colour is concentrated in mobile streaks of red and orange that appear to flicker across the surface as the stone is rotated. The term is descriptive rather than mineralogically precise, and it is applied to material from various sources whose body colour and play-of-colour patterns produce the characteristic effect.

Optical character

Play-of-colour in precious opal is produced by the diffraction of white light through the regular array of silica spheres that constitute the opal-AG and opal-CT structures. The size of the spheres governs the wavelength range diffracted: spheres of approximately 150 nanometres diameter produce blue and green flashes, while spheres of approximately 350 nanometres produce red. Flame opal as a category is characterised by a relatively uniform sphere size in the larger end of the play-of-colour range, producing predominantly red and orange diffraction.

The flame appearance, distinct from a static red broadflash, requires that the play-of-colour appears in moving streaks rather than as discrete patches. This depends on the geometry of the play-of-colour layers within the stone. Where the diffraction layers are thin, parallel, and tilted with respect to the cut surface, rotation of the stone produces the rolling effect. Mexican fire opal exhibits this effect notably in higher grades. Material from Andamooka in South Australia, from the Wollo deposits in Ethiopia, and from certain pockets at Lightning Ridge in New South Wales also produces flame patterns.

Sources and varieties

Mexican opal from the Magdalena and Queretaro fields is the historical type for the term flame, and the orange-red body colour of fire opal often combines with the red play-of-colour to produce stones that read as luminous from across a room. The body of Mexican fire opal is hyalite to translucent and the play-of-colour, where present, is generally subordinate to the body colour. The flame designation in Mexican opal applies particularly to material in which the play-of-colour streaks are extensive enough to register as the principal optical feature.

Wollo opal from Ethiopia, mined commercially since the 2008 discovery near the town of Wegel Tena in the northern highlands, includes flame-pattern stones in which the white or transparent body shows broad red streaks. The Wollo material is hydrophane, meaning it absorbs water and changes appearance temporarily on immersion, and this characteristic must be considered in setting and care. Some Wollo flame opal exhibits play-of-colour in concentric or radial patterns that move differently from the linear flames of Mexican material.

At Lightning Ridge, flame patterns are found in both white and black opal, although the predominant Lightning Ridge optical signature is the multicolour broadflash and pinfire patterns. Flame in Lightning Ridge black opal is highly valued because of the contrast between the dark body and the bright red streaks.

Grading and pricing

Flame opal is graded along the same criteria as other precious opal: brightness of play-of-colour, dominance of red in the spectrum (red being the rarest and most highly priced colour), pattern, body tone, and clarity. The Australian opal trade, formalised through the Australian Opal Industry Association classification system, uses a brightness scale from B1 (very bright) to B7 (very dull) and a body tone scale from N1 (jet black) to N9 (very light). Flame opal would typically be evaluated within these standard categories.

Pricing for flame opal at the high end follows the same logic as other red-dominant play-of-colour material. A black opal with a strong red flame pattern, of one carat in weight and bright at B1 to B2, can reach prices in the tens of thousands of dollars per carat at retail. Mexican flame opal, generally lower in body tone, achieves more modest prices but with significant variation by quality. Wollo flame opal, which entered the market relatively recently and remains under price discipline because of supply, is generally less expensive than equivalent Australian material.

Care

Opal is hydrated silica with water content typically between three and ten per cent by weight. The stones are sensitive to dehydration, to thermal shock, and to mechanical impact at the lower hardness range of 5.5 to 6.5 on the Mohs scale. Flame opal does not differ from other opal in this respect. Doublet and triplet construction, in which a thin slice of opal is cemented to a backing and sometimes capped with a clear cover, is common in commercial flame opal jewellery and should be disclosed in the description. Solid flame opal is the more highly valued category.