Contra-luz Opal

The opal whose fire lives in transmitted light

Gem varietiesView in dictionary · 1,290 words

Contra-luz opal is a rare and optically distinctive variety of precious opal in which the characteristic play-of-colour — the spectral diffraction phenomenon that defines precious opal — manifests primarily or exclusively when light passes through the stone rather than reflecting from its surface. The term derives from the Spanish contra luz, meaning "against the light," and describes precisely the viewing condition required to unlock the gem's chromatic display. In reflected light, a contra-luz opal may appear deceptively plain: a pale, translucent to near-transparent body with little obvious colour. Held up to a lamp or window, however, the interior ignites with shifting greens, blues, reds, and violets of remarkable saturation. This inversion of the usual opal experience makes contra-luz stones objects of genuine fascination among collectors and gemmologists alike.

Optical Mechanism

All precious opal owes its play-of-colour to the diffraction and interference of visible light by a three-dimensional lattice of amorphous silica spheres (SiO₂·nH₂O) arranged in a regular, close-packed array. When sphere diameter and spacing approximate the wavelengths of visible light (roughly 400–700 nm), the lattice acts as a diffraction grating, separating white light into its spectral components. The specific colours produced depend on sphere size: smaller spheres yield violet and blue; larger spheres produce red and orange.

In conventional precious opal — whether white, black, or crystal — this diffraction is observed in reflected light because the body of the stone is sufficiently opaque or translucent to scatter and return light toward the viewer. In contra-luz opal, the silica sphere arrangement and the overall transparency of the host material are such that diffracted wavelengths are transmitted more efficiently than they are reflected. The stone's body tone is typically colourless, very pale grey, or faintly yellowish, and its diaphaneity ranges from translucent to genuinely transparent. Light entering the stone traverses the diffracting lattice and emerges on the far side carrying the spectral colours, which the eye perceives when the light source is positioned behind or beside the gem. Some contra-luz specimens exhibit a weak play-of-colour in both transmission and reflection, but the transmitted display is invariably the more vivid.

Origin and Geology

The great majority of contra-luz opals documented in the gem trade originate from Mexico, and within Mexico principally from the state of Jalisco. The Jalisco deposits — centred on the municipality of Magdalena — have produced precious opal of many types since at least the nineteenth century, including the celebrated Mexican fire opal, crystal opal, and contra-luz material. The opals occur as nodules, veins, and cavity fillings within rhyolitic volcanic rocks of Oligocene to Miocene age. Silica-rich hydrothermal or meteoric fluids percolated through fractured rhyolite, depositing opal gel that subsequently ordered itself into the sphere arrays responsible for play-of-colour.

The particular transparency that enables the contra-luz effect appears to be favoured by a relatively low concentration of micro-inclusions and a high degree of silica purity within the deposited gel. Jalisco material is generally noted for its clarity compared with Australian sedimentary opal, and this geological character predisposes it to producing contra-luz specimens. Smaller quantities of contra-luz-type opal have been reported from other Mexican states, including Querétaro, though Jalisco remains the primary commercial source. Contra-luz material has also been noted occasionally from Ethiopian opal deposits (Wollo Province), where the hydrophane opal's high transparency can, in select specimens, produce a transmitted play-of-colour, though Ethiopian material is not the canonical source for the variety.

Physical and Chemical Properties

Contra-luz opal shares the fundamental properties of all precious opal:

Composition: Hydrated silicon dioxide, SiO₂·nH₂O, with water content typically 3–10% by weight in Mexican material.
Crystal system: Amorphous (no crystalline structure).
Refractive index: Approximately 1.37–1.47, varying with water content; typically near 1.45 for Mexican opal.
Specific gravity: Approximately 1.98–2.20, somewhat lower than many silicate gems owing to the hydrated, porous microstructure.
Hardness: 5.5–6.5 on the Mohs scale — relatively soft and susceptible to scratching and abrasion.
Cleavage: None; fracture is conchoidal.
Diaphaneity: Translucent to transparent, a defining characteristic of contra-luz material.
Body colour: Colourless, very pale grey, pale yellow, or faintly orange; the body tone must be light enough to permit significant light transmission.

Because opal is amorphous and hydrated, it is susceptible to crazing (surface cracking) if subjected to rapid temperature changes or prolonged desiccation. Mexican contra-luz opal, like other Mexican precious opal, tends to be somewhat more stable than Australian sedimentary opal in this respect, though care in storage and wear remains advisable.

Cutting and Presentation

The cutting of contra-luz opal presents challenges and creative opportunities not encountered with conventional precious opal. Because the play-of-colour is a transmitted phenomenon, the lapidary must consider not only the outline and proportions of the finished stone but also the path of light through it. Cabochons are the most common form: a domed top concentrates and directs transmitted light effectively, and the smooth surface minimises surface reflection that might compete with the transmitted display. However, contra-luz opal is also faceted — an unusual treatment for any opal — because faceting can enhance the stone's transparency and create multiple transmission angles that shift the visible colours as the stone is moved relative to a light source.

Thin slabs or plates are sometimes fashioned to maximise transmission, and these may be set in open-backed or bezel settings that allow light to enter from below. Jewellers working with contra-luz material occasionally design pieces specifically to exploit back-lighting — pendants worn near a window, for instance, or rings positioned to catch raking light. The gem rewards thoughtful setting in a way that few other stones do.

In the Trade and Among Collectors

Contra-luz opal occupies a specialist niche within the broader opal market. It is not widely stocked by general jewellery retailers and is most commonly encountered through dealers who focus on Mexican gemstones, collector-oriented gem shows, and auction houses handling unusual mineral specimens. Pricing is highly variable and depends on the intensity and colour range of the transmitted play-of-colour, the clarity and transparency of the body, the size of the stone, and the quality of the cut. Stones displaying a full spectral range — red, orange, yellow, green, and blue — in strong, shifting patches command the highest premiums among collectors.

Because contra-luz opal is not widely known to the general public, it is sometimes undervalued in non-specialist settings. Conversely, its rarity and the theatrical quality of its transmitted display have made it a favourite of gem collectors who prize optical curiosities alongside conventional beauty. It is regularly discussed in the context of collector gems alongside phenomena such as alexandrite's colour change and cat's-eye chrysoberyl's chatoyancy — stones whose appeal depends on a specific interaction between light and structure rather than on colour alone.

No significant treatments specific to contra-luz opal are in widespread commercial use, though the general opal treatments — impregnation with colourless resin or oil to improve stability and apparent clarity — are occasionally applied to Mexican opal broadly and may affect contra-luz material. Buyers seeking untreated stones should request disclosure and, for significant purchases, laboratory reports from recognised gemmological laboratories.

Identification and Separation

Identifying contra-luz opal is straightforward for any gemmologist familiar with the variety: the defining test is simply to transmit light through the stone and observe whether play-of-colour appears or intensifies. Standard gemmological instruments — refractometer, spectroscope, specific gravity measurement — confirm the opal identification. Separation from glass or synthetic opal simulants follows conventional opal testing protocols, including examination under magnification for the characteristic "snakeskin" or "chicken-wire" pattern of play-of-colour patches seen in Gilson synthetic opal, and refractive index measurement. Raman spectroscopy, available at specialist laboratories, can confirm the amorphous silica composition and distinguish natural opal from simulants definitively.