How Are Opals Formed?
Born of Water, Time and Silence
How opals form is above all a story of patience: the finished stone you hold in your hand took shape over spans of time our imagination simply cannot hold. Those long ages also decide why some stones glow and others stay milky.
No two opals are alike, and that starts at birth. Some grow in the Australian desert, some in volcanic rock in Ethiopia, some where a piece of wood or a shell lay in the distant past. We will walk the whole way with you, from a single drop of water to the finished play of color on your hand.
What is an opal actually made of?
An opal is solidified silica gel: amorphous silicon dioxide with water locked inside, chemically SiO₂·nH₂O. “Amorphous” is the key word here. Unlike quartz, sapphire or diamond, an opal has no ordered crystal lattice. At heart it is a gel that grew ancient, and that is exactly what makes it so singular.
How does an opal form, step by step?
Millions of years pass between the first drop of water and the finished stone. The path runs through five stages, and with each one the stone that no one will ever find twice comes a little closer.
Water takes up silica
Over long stretches of time, rain seeps through soils and rock rich in silicon dioxide and dissolves silica out of them. What remains is a thin, mineral-rich solution that slowly travels downward.
The solution gathers in cavities
It works its way into cracks, fissures and crevices, often ones opened up by tectonic movement, weathering rock or decaying fossils. There it comes to rest.
The water evaporates
Over long dry phases the water escapes bit by bit. What stays behind is a soft silica gel that fills the cavity completely, the raw material of the later stone.
The spheres fall into order
Tiny spheres of silicon dioxide form inside the gel. If they are all the same size and stack into a regular lattice, the foundation for the play of color is laid. If that order never comes, the result is a quiet, milky opal.
Over millions of years the stone solidifies
Under pressure the gel hardens very slowly into solid opal. Just how slowly can only be estimated: the great Australian deposits sit in rock from a Cretaceous inland sea, more than a hundred million years old. Some sources reckon on millions of years for just a few centimeters. The stone on your hand is older than the dinosaurs.
Why do opals flash in every colour?
The play of colour does not come from dyes, but from pure light. The tiny silica spheres sit in a regular lattice and act like a natural diffraction grating: they split incoming white light into its spectral colours, much like a rainbow or the back of a CD.
The decisive point: the colour you see depends on the size of the spheres. Small spheres, around 150 nanometres, diffract short-wave light and show blue and violet. Large spheres, about 350 nanometres, diffract long-wave light and show orange and red. That is why red fire in opal is so rare and so sought after: it needs the largest, perfectly ordered spheres.
This is where precious opal parts from common opal. When the spheres sit in order, the flashing play of colour appears. When they vary in size and sit in chaos, the stone stays milky and quiet. Both are real opal. Nature simply stacked more cleanly in one case than in the other.
| Sphere size | Visible colour | How common |
|---|---|---|
| largest (~350 nm) | Red | rarest |
| large (~280 nm) | Orange | rare |
| larger (~250 nm) | Yellow | less common |
| medium (~220 nm) | Green | common |
| smaller (~200 nm) | Turquoise | common |
| small (~190 nm) | Blue | most common |
| smallest (~150 nm) | Violet | often with blue |
Which colour reveals which structure?
Tap a colour.
Do all opals form in the same way?
No. Opal can form in four very different ways, and the path shapes the finished stone, from colour through transparency to origin. That is why an Australian black opal and an Ethiopian Welo opal look so different, even though both are called "opal".
1 · Sedimentary
The most common path. Water dissolves silica from sandstone, seeps into cavities and evaporates over geological time. That is how most Australian deposits formed, including white opal, crystal opal and the coveted black opal. In the field it looks like this: in Coober Pedy, pale chalk-white spoil heaps rise out of the desert. The rough is dusty and almost like chalk. Only when you tilt it does play-of-colour flash in the crack. In Lightning Ridge the same process sits in dark claystone, which is why the stone looks black there.
2 · Volcanic
Silica-rich water fills cavities in volcanic rock such as rhyolite or tuff. That is how Ethiopian Welo opals with their lively colours form, as well as the orange-red fire opals from Mexico. In Querétaro and Magdalena the host rock is brick-red, porous rhyolite. Fire opal sits in old gas bubbles like honey in a comb: clear, orange to red, often transparent. Around it: cactus, dry hills, volcanic highlands. Not desert sandstone, but cooled lava.
3 · Organic (petrification)
Here silica replaces organic material piece by piece. That is how wood opal and opalised fossils form, in Lightning Ridge even opalised shells and dinosaur bone. The rough often still looks like the original: growth rings in wood, the curve of a shell, the pores of a bone. Only the tissue has been replaced by opal, and it lights up green and blue when you tilt it.
4 · Boulder
In Queensland, around Quilpie and Yowah, opal fills fine cracks in ironstone nodules. The host rock is rust-brown to black-brown and stays as the back of the stone. On the field the nodules look like ordinary brown ironstone chunks in an open cut. Only the crack shows the play of colour. That is why a boulder opal is brown: not the opal, but the ironstone it grew in.
Each of these paths produces its own opal types, from black opal through Ethiopian Welo opal to boulder opal and fire opal. An overview of all opal types is in the lexicon.
About 95 percent of the world's precious opals come from Australia. At Liberty Angels you will find a wide variety of every kind of opal.
Grown in the earth or made in a lab?
Since the 1970s there have been synthetic opals modelled on the natural sphere structure, made in weeks instead of millions of years. There are also straight imitations in plastic or glass. Neither is "wrong" in the sense of worthless, but they did not grow in the earth. This is how you tell grown from made:
Naturally grown
over millions of years in the earth
- irregular, living play of colour that moves with the angle of view
- fine inclusions and small natural marks
- cool on the skin, feels "stony"
- every piece is unique
Labor & Imitat
made or imitated in weeks
- often a pattern that is too regular, sometimes a column or snakeskin effect
- strikingly flawless, with no trace of nature
- plastic feels warm and light
- many stones look identical
The full guide with every detail is here: How to spot a real opal: the 5 surest signs.
Find your opal
Handmade, GSB-certified, in silver and gold. Every stone from the depth of time.
Common questions on how opals form
How do opals form?
Opals form when silica-rich water seeps into rock cavities, evaporates and leaves behind tiny silicon-dioxide spheres. Over millions of years these harden into solid opal.
How long does it take for an opal to form?
Millions of years. The Australian deposits sit in rock more than a hundred million years old.
What is an opal made of?
From amorphous silicon dioxide with trapped water (SiO₂·nH₂O). The water content is usually between 3 and 21 percent.
Why do opals have a play of colour?
Tiny spheres of equal size sit in a regular lattice and diffract light like a rainbow. Sphere size decides the colour: small for blue, large for red.
Where do most opals form?
About 95 percent of precious opals come from Australia. Other important sources are Ethiopia for Welo opals and Mexico for fire opals.
Can opals be made artificially?
Yes. Since the 1970s there have been synthetic opals that copy the sphere structure.
How does wood opal form?
Through petrification: silica gradually replaces the organic material of wood or other fossils until an opalised likeness remains.
If you are unsure or have questions, you are welcome to contact us at any time, even if you did not buy the opal from us. Contact form, email cs@libertyangels.de or call +49 171 1603048.
Verified authenticity
by Liberty Angels
Only natural, genuine opals. Each certificate is linked online to your order, so it is tamper-proof.
Written by Anton Kehler
Gründer FAERELLI & Liberty Angels
