What Is an Opal? The Gemstone That Is Not a Crystal
Opal knowledge · Basics
The gemstone that is not a crystal
An opal is the great exception among the major gemstones. Diamond, ruby, sapphire and emerald are crystals with a fixed lattice. The opal has none. It is amorphous, it contains water, and its colours are not held inside it as pigment but are created anew from light every time. That is not a detail for mineralogists but the reason behind everything that defines it.
The essentials: Opal consists of tiny spheres of silica gel and contains water. It has no crystal lattice, which is why it cannot be cleaved and only faceted to a limited degree. Its colours come from light diffraction on those spheres, which is why no opal looks like another.
Why is an opal not a crystal?
Almost all gemstones are crystals: their atoms sit in a strictly ordered lattice that repeats in every direction. With opal that is not the case. It forms when silica rich water seeps into cavities and dries out over a very long time. What remains are tiny spheres of silica gel, stacked like oranges in a crate.
Fixed atomic lattice, defined cleavage directions, uniform optical properties. Can be cut and faceted according to fixed rules.
No lattice, no cleavage direction. That is why it is almost always cut round, as a cabochon, because facets tend to cut up the play of colour rather than emphasise it.
How much water is in an opal?
An opal consists of a measurable share of water, usually in the low to middle single digit percentage range, and considerably more in some pieces. No other well known gemstone carries this much water.
Which is why it dislikes heat
If an opal dries out too quickly, fine cracks can form. Saunas, radiators and the car dashboard in summer are the classic danger spots.
Which is why it is lighter
At the same size an opal weighs less than a quartz or a sapphire. Anyone holding both will feel the difference.
Which is why hydrophane exists
Some opals, above all from Ethiopia, absorb water and briefly turn transparent. Once dry, the play of colour returns.
More on the Ethiopian stones and how they behave is in the article on Welo opal.
Where does the colour come from if there is no pigment?
A ruby is red because chromium sits in the crystal. An emerald is green because it contains chromium or vanadium. With opal no pigment is involved at all. Its colours come purely from structure.
Small spheres diffract short wave light: blue and violet. They are common, which is why blue is the most widespread opal colour.
Large spheres diffract long wave light: orange and red. They are rare, which is why red is the most expensive.
Because the colour depends on the angle at which light meets the spheres, it changes as soon as you move the stone. That is the difference between a stone you look at and one you watch. The physics behind it is in the play of colour of the opal, the individual colours in the opal colours at a glance.
Why are no two opals alike?
The size of the spheres, their arrangement, the thickness of the colour bearing layer, the base tone of the surrounding material: all of this forms over millions of years under conditions that never repeat exactly. Two opals from the same rough piece already show different patterns.
With diamonds, sameness is the mark of quality. With opal, difference is the property.
This has a practical consequence many underestimate: a pair of earrings with two matching opals is far more demanding than a pair with two diamonds. The stones have to be searched for, they cannot simply be ordered.
What does that mean for you day to day?
| Property | Consequence in daily wear |
|---|---|
| No crystal lattice | usually cut as a cabochon, rarely faceted |
| Water content | no sauna, no radiator, no ultrasonic bath |
| Mohs hardness 5.5 to 6.5 | Mohs 5.5 to 6.5, so store it apart from harder stones |
| Colour through structure | looks different in every light, photos never show the whole piece |
| Every stone unique | no buying again, no replacement if lost |
How to treat an opal correctly is in opal care. How to spot genuine goods is in how to spot a real opal.
If you want to know which opal suits you: the opal finder takes you to a selection in four steps. Or browse straight through the opal jewellery.
What this means for you when wearing it
You do not need to be a mineralogist to handle an opal properly. Three things are enough: take it off before you clean or exercise. Do not leave it next to the radiator or in the car in the sun. And store it away from harder stones, a diamond in the same box will scratch it. Keep to that and your opal will last longer than you think.
Common questions
What is an opal?
A mineraloid of tiny silica gel spheres with a water content. Unlike diamond or sapphire it has no crystal lattice, which is why it strictly counts as a mineraloid rather than a mineral.
Why is opal not a crystal?
Because its building blocks do not sit in a regular atomic lattice. It forms from dried silica solution, not through crystal growth.
How much water does an opal contain?
Usually a few percent, considerably more in some pieces. That is why it reacts sensitively to strong heat and very dry air.
Why does opal have so many colours?
Because the colours do not come from pigments but from light diffraction on the silica gel spheres. Their size determines which colour you see.
Why is opal cut round?
Because it has no crystal lattice, facets achieve little. A round cut, the cabochon, shows the play of colour as one connected surface.
Are there two identical opals?
No. Even two stones from the same rough piece show different patterns.
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
