Gem library
Opal
Hydrated silica that holds the rainbow
Opal is not a crystal but a stack of hydrated silica microspheres. When those spheres are of regular size they diffract light like an optical grating: that is the play of colour, a phenomenon unique in the mineral kingdom.
- Hardness (Mohs)
- 5.5 – 6.5
- Density
- 1.98 – 2.25
- Crystal system
- Amorphous — stacked silica spheres
- Refractive index
- 1.44 – 1.46
- Cleavage
- None
Mineralogical identity
- Family
- Hydrated amorphous silica
- Species
- Opal
- Variety
- Black opal, white opal, fire opal, water opal, hyalite, common opal
- Chemical formula
- SiO₂·nH₂O (3 to 21 % water)
- Crystal system
- Amorphous — stacked silica spheres
- Classification
- Semi-precious stone
- Rarity
- Rare
- Indicative price
- €5 – 15,000/ct (Lightning Ridge black opal)
Etymology
From the Sanskrit upala, «precious stone», via the Greek opallios and Latin opalus.
Physical & optical properties
- Hardness (Mohs)
- 5.5 – 6.5
- Density
- 1.98 – 2.25
- Refractive index
- 1.44 – 1.46
- Birefringence
- None
- Cleavage
- None
- Fracture
- Conchoidal
- Lustre
- Vitreous to resinous
- Diaphaneity
- Transparent to opaque
- UV fluorescence
- Strong
- Pleochroism
- None
- Colours
- multicoloured, white, black, orange, blue
Formation, geology & history
The physics of play of colour
Precious opal consists of silica spheres 150 to 400 nanometres across, stacked in a regular lattice with water between them. Those spheres diffract light like a natural photonic crystal: each ordered domain returns a colour depending on its sphere size and the viewing angle. Small spheres give blue and violet, large ones red and orange — which is why red, requiring the largest spheres, is the rarest and most valued. When the spheres are of irregular size no diffraction occurs: the result is common opal, called potch, without play of colour and without gem value.
Black, white and fire opal
Classification rests on the body colour. Black opal, whose dark ground makes the colours explode, comes practically only from Lightning Ridge in New South Wales and represents the top of the market — up to €15,000/ct for a red-dominant piece. White or milky opal, from Coober Pedy, is far more plentiful. Water (or crystal) opal is transparent with the play of colour in suspension. Mexican fire opal, orange to red, takes its colour from iron and often has no play of colour at all. Ethiopian opals from Wollo, which appeared in 2008, have a hydrophane structure: they absorb water and become temporarily transparent, which disturbs their play of colour.
Crazing, the collector's dread
Opal contains up to 21 % water. If it dehydrates — heat, dry air, prolonged exposure — it cracks into a network of fine fissures called crazing, an irreversible phenomenon that destroys the stone. Australian mine opals, stable, are little affected; Ethiopian and some Brazilian opals far more so. Precautions: never heat it, never leave it in the sun, never store it in an over-dry safe, and avoid abrupt humidity changes. Some conservators keep their opals in slightly damp cotton, a debated but widespread practice.
How to recognise it
- Mobile play of colourThe colours must move and change as you tilt the stone, in patches rather than spangles. A fixed sparkle suggests glass or a synthetic opal.
- Low density2.0 to 2.2: opal is markedly lighter than quartz. A heavy iridescent stone is not an opal.
- Columnar or plate structureSeen from the side, natural opal shows irregular three-dimensional colour domains; synthetics have a regular columnar structure known as «snakeskin».
Imitations, treatments & confusions
- Synthetic opal (Gilson, Kyocera)Regular columnar structure visible in profile, play of colour too perfect and too dense, «lizard-scale» patterns. Often more regular than any natural stone.
- Doublets and tripletsA thin slice of opal glued to a black backing (doublet), sometimes capped with quartz (triplet). Look at the girdle in profile: the bond line is visible. The value is far lower.
- Opalite and iridised glassMilky glass with a uniform bluish sheen, no mobile play of colour. Bubbles under the loupe, warmth in the hand.
Deposits & origins
Australia (95 % of world production: Lightning Ridge for black opal, Coober Pedy for white, Andamooka), Ethiopia (Wollo, contact opals found in 2008), Mexico (Querétaro fire opal), Brazil, Honduras, Peru, Slovakia (the historic Dubník deposit)
Care & precautions
- Never hot water, never prolonged soaking, especially on hydrophane Ethiopian opals, which absorb liquids and cloud over.
- A soft, slightly damp cloth. No household products, no solvents, no soap.
- Ultrasonic cleaners, steam and heat are absolutely forbidden: immediate and irreversible crazing risk.
- Avoid over-dry air, direct sun and heated safes. Never wear an opal to a swimming pool or sauna.
Traditions & crystal healing
- Chakras
- Varies · crown
- Zodiac signs
- Libra · Cancer · Pisces
- Element
- Water
- Month
- October
Long considered unlucky in Europe — a reputation born of a Walter Scott novel in 1829, not of ancient tradition — opal is on the contrary a stone of creativity and inspiration in older traditions. It is credited with opening the imagination and easing emotional flow.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Why is my opal crazing?
That is crazing: dehydration of a stone that contains up to 21 % water. The process is irreversible. Avoid heat, sun, dry air and abrupt humidity changes.
Natural opal or doublet?
Look at the girdle in profile under a loupe: a doublet shows a clean bond line between the opal slice and its black backing. A natural opal is homogeneous through its whole thickness.
Can an opal be worn every day?
It is risky. Hardness 5.5-6.5, sensitivity to water, heat and dehydration: keep it for jewellery with little exposure, as a pendant or an evening ring in a protective setting.