silicate
Opal (Precious Opal)
Hydrated amorphous silica exhibiting play-of-colour from diffraction; Mohs 5.5–6.5.
Opal is a hydrated amorphous silica mineraloid (SiO2·nH2O) that lacks the long-range crystal order of true minerals. It rates 5.5–6.5 on the Mohs scale. Precious opal — the variety displaying play-of-colour — consists of stacked spheres of amorphous silica (diameter 150–300 nm) arranged in a regular face-centred cubic or hexagonal packing. Light diffracts from these sphere layers, producing spectral colours whose wavelength depends on sphere size and array periodicity. Australia produces the majority of the world's precious opal; New South Wales (Lightning Ridge) produces the prized black opal. Ethiopia's Wollo province has emerged as a major opal source since the 2000s.
Quick facts
- Item type
- Mineral
- Mineral class
- silicate
- Mohs hardness
- 6
- Crystal system
- amorphous
- Chemical formula
- SiO2·nH2O
- Color range
- white, black, orange, red, yellow, green, blue, multicolour-play-of-colour
- Notable localities
- Lightning Ridge, New South Wales, Australia (black opal); Coober Pedy, South Australia (white/light opal); Mintabie, South Australia (black and semi-black opal); Wollo Province, Ethiopia (hydrophane precious opal); Queretaro, Mexico (fire opal)
Play-of-Colour: Silica Sphere Diffraction
The play-of-colour in precious opal arises from a regular arrangement of silica spheres with diameters between 150 and 300 nm. Electron microscopy studies (Sanders & Darragh, 1967) confirmed the sphere-stack model. When white light enters the stone, it diffracts from the sphere layers according to Bragg's law: nλ = 2d sinθ, where λ is the diffracted wavelength, d is the sphere-layer spacing, and θ is the angle of incidence. Sphere diameters of ~200 nm produce green; ~250 nm produce red. Rotation of the stone changes θ, shifting the dominant diffracted wavelength — this is the characteristic colour-shifting ('rolling') play-of-colour. Common opal lacks the ordered sphere arrangement and shows no play-of-colour. The water content (typically 3–21 wt%) fills inter-sphere spaces and must be maintained to preserve play-of-colour; dehydration can cause crazing.
Black Opal and Body Colour Classification
Opal is classified by body colour: black opal (N1–N4), dark opal (N5–N6), light opal (N7–N9), and crystal/water opal (highly transparent). Lightning Ridge, New South Wales, is the sole significant source of high-quality black opal. The dark body colour (from trace carbon and iron oxide) creates a high-contrast background that intensifies play-of-colour, similar to how dark velvet enhances reflected light from diamonds. Premium black opal with full spectral play-of-colour from Lightning Ridge is among the highest unit-value gemstones. Boulder opal from Queensland occurs as seams in ironstone matrix; the ironstone back is retained and provides a natural dark background. Matrix opal (Andamooka) consists of opal dispersed through a porous rock matrix.
Ethiopian Opal and Hydrophane
Wello (Wollo) Province opal from the Afar region of Ethiopia, commercialised from around 2008, occurs in rhyolitic volcanic rock and is often hydrophane — a type of opal with significant porosity that absorbs water readily, temporarily becoming transparent and altering play-of-colour. Hydrophane opals can absorb oils, perfumes, and dyes that change their appearance; they must be kept dry and away from cosmetics. Ethiopian opal spans a range from common to precious; the most valuable material shows vivid multicolour play-of-colour similar to Australian material. Mexican fire opal (Queretaro) is a separate opal type: translucent to transparent orange-red opal from volcanic tuff with little or no play-of-colour.
Sources & further reading (3)
- gemological-institute — accessed 2026-05-08
- encyclopedia — accessed 2026-05-08
- mineral-database — accessed 2026-05-08
Frequently asked questions
What causes opal's play-of-colour?
Play-of-colour results from light diffraction by regularly stacked silica spheres inside precious opal. The spheres — 150 to 300 nm in diameter — are packed in a face-centred cubic or hexagonal arrangement. When white light enters the stone, each pair of adjacent sphere layers acts as a diffraction grating, reinforcing specific wavelengths (colours) depending on sphere size and viewing angle. The colour changes as you tilt the stone because the diffraction angle changes. Sphere diameter determines the dominant colour: red requires larger spheres (~260 nm), blue-green smaller (~150–200 nm).
Can opal dry out and crack?
Yes. Opal contains a variable amount of water (typically 3–21 wt%) in the pores between silica spheres. If this water evaporates rapidly — through prolonged low humidity, storage in an airtight container with silica gel, or exposure to heat — the matrix shrinks unevenly and surface or internal cracks (crazing) can develop. This is more common in opals with higher water content and in stones that formed in environments with significant water infiltration cycles, such as some Ethiopian hydrophane opals. Australian opals from stable geological settings are generally more stable. Crazing is irreversible; affected stones cannot be repaired.
Is Australian opal different from Ethiopian opal?
Both produce precious opal with play-of-colour, but they differ in geology, body colour, and stability. Australian opals (South Australia, New South Wales) formed in sedimentary environments where silica-bearing groundwater filled cavities in sandstone or ironstone; they are generally stable and non-hydrophane. Ethiopian Wollo opals formed in volcanic rhyolite and are often hydrophane — they absorb water, temporarily changing transparency. Ethiopian opals tend to have higher water content and can be less stable in very dry conditions. High-quality examples from both regions are gemologically equivalent; provenance is traceable by inclusion type and trace element chemistry in laboratory reports.