Cracking opal's origin took ten years; the key evidence came from Mars
Central Australia's ancient sea left no limestone to buffer acid weathering, allowing volcanic basalt to transform into silica gel—a mechanism first confirmed by opal discoveries on Mars, then worked backward to explain terrestrial opal origins.
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A ten-year-old's fascination with opal mining became his life's research
At ten, Patrice Rey saw a documentary about Coober Pedy's opal mines, mesmerized by miners striking it rich and building underground mansions with swimming pools. At fourteen he wrote to Australia's embassy asking about immigration; they actually sent him information. Years later, a call from Lightning Ridge's opal miner association ignited the research that would dominate his career.
— Patrice ReyOpal is not truly crystalline; it's structured silica gel that diffracts light
Most opal is worthless potch opal—gray and nondescript. Only a few percent achieve precious status because their silica spheres are uniform and aligned to diffract light into iridescence, and each piece's pattern is completely unique (unlike diamonds, which look similar). Crucially, opal isn't crystalline at all. It began as aqueous gel suspension where silica spheres grew slowly. Random sphere sizes yield dull potch; uniform sizes and ordered arrangement yield the play-of-color that defines precious opal.
— Patrice ReyUnlike diamonds, opal forms without extreme pressure or temperature
Chemically, opal is straightforward: roughly 10% water, the rest silica. It's the only hydrous gemstone because it forms near Earth's surface, not in the deep crust. Diamonds demand depths beyond 150 km and temperatures exceeding 1000°C. Opal needs neither. Patrice notes you could actually synthesize opal in your kitchen with basic equipment.
— Patrice ReyCentral Australia's ancient sea lacked limestone, enabling opal to form through acid weathering
It took Patrice ten years to assemble the answer. Precious opal requires two things: silica gel released by weathered volcanic rock, and strong acidity to break feldspar into soluble silica. Limestone normally blankets Earth's surface and neutralizes acid, blocking this reaction. Central Australia's ancient shallow inland sea, uniquely, had no limestone buffer at all. Without it, sulfuric acid from pyrite oxidation persisted for eons, deeply acid-etching basalt into silica gel and enormous quantities of iron oxide—the origin of Central Australia's distinctive red landscape.
— Patrice ReyOpal deposits on Mars provided the key evidence for Earth's opal formation
In 2008, scientists discovered opaline silica across Mars's surface. Mars is also basalt, and roughly three billion years ago it was wet with flowing rivers and lakes, then lost its water. The exposed basalt underwent intense acid weathering from acidic atmosphere, releasing opaline silica—the identical process to Central Australia's evaporated inland sea undergoing acidic weathering. Mars's red color and Central Australia's Red Center both stem from the same iron-oxide weathering reaction.
— Patrice ReyBacteria create fossilized gems by slowly replacing organic matter with silica
Some opal fossils preserve detail down to the cellular level. Anaerobic bacteria decomposing dead organisms in oxygen-starved mud produce pyrite (fool's gold). When this pyrite later oxidizes in air, it releases sulfuric acid and silica, which gradually replaces the fossil's original carbonate structure with silica dioxide molecule by molecule—a process that must occur extremely slowly to preserve cellular-level detail.
— Patrice ReyOpal mines operate as small-scale ventures in areas barely larger than a tennis court
Opal mining differs fundamentally from diamond mining. Diamonds require massive capital and excavating enormous volumes of rock to find tiny crystals—a capital-intensive industry. Opal mines are typically run by two or three people with shovels and picks, on sites roughly 50 by 50 meters. A bulldozer is a luxury. The environmental footprint is far smaller. However, Queensland and other states require miners to restore their sites, and rising diesel and insurance costs are making this cottage industry increasingly difficult to sustain.
— Patrice ReyOpal requires careful handling but repolishing can restore most surface damage
On the Mohs hardness scale, diamonds score 10; opal scores only 5 to 6—barely harder than glass. The American Gem Society explicitly rates opal's toughness as 'poor to fair'. Daily wear risks cracking from sudden temperature changes and direct sunlight. Australian opal is non-hydrous (safe in water); Ethiopian opal is hydrous and yellows or clouds if soaked (it recovers slowly with air-drying in shade). The silver lining: most surface scratches can be repolished smooth, unless deep oil infiltration has permanently yellowed it—that's nearly impossible to reverse.
— Ally WardIn their own words · checked verbatim
Precious opal is a very small percentage of opals, a few percent. And that's why they are gemstones. That's why they are precious. It's because they are much rarer.
Patrice Rey13:15
It forms at the beginning as a gel, an unconsolidated fluid. And in that fluid, what we observe is the crystallization of silica sphere.
Patrice Rey14:15
Diamond, for instance, if you want to have a diamond, the diamond forms at least 150 kilometers depth, at a temperature, you know, in excess of 1,000 degrees. But no, opal, you can make opal in your kitchen.
Patrice Rey16:23
And this is when the pin dropped for me. I realized that there is almost no limestone in Central Australia. Although it was a very shallow inland sea, there was absolutely no limestone, which is very unusual for this kind of inland sea.
Patrice Rey30:41
And by the way, we have the red center and the color is the same on Mars.
Patrice Rey37:00
A lot of miners think that this is very recent, you know, 30 million years. In my research, you know, I placed the bulk of formation of opals in between 93 to 60 million years.
Patrice Rey42:04
toughness is very poor to fair, making opal a gem that is suitable for jewelry but requires care when wearing
Ally Ward59:00
Figures
| Opal's water content | up to approximately 10% | 16:23 |
| Diamond formation depth and temperature | at least 150 km deep, over 1000 degrees Celsius | 16:23 |
| Silica sphere diameter in yellow/orange/red opal | approximately 250–400 nanometers | 18:24 |
| Silica sphere diameter in blue opal | 200 nanometers or less | 18:24 |
| Opal's Mohs hardness | 5–6 (diamond is 10) | 59:00 |
| Typical single opal mine area | 50 m × 50 m | 51:28 |
| First discovery of opal at Earth's surface in Australia | 1915 | 11:13 |
| Discovery of opaline silica on Mars | 2008 | 33:55 |
| Mars had liquid water | approximately 3 billion years ago | 34:58 |
Glossary
- potch opal
- Dull, non-iridescent, valueless common opal, distinguished by the absence of color play
- opaline silica
- Non-crystalline silica residue left after volcanic rock undergoes intense acid weathering
- hydrophane
- Porous variety of opal that absorbs water and becomes cloudy; typical of Ethiopian opal
- native title
- Australian legal recognition of Indigenous peoples' rights to land based on continuous traditional use
- doublet/triplet opal
- Composite gemstone created by adhering thin opal to backing or protective layer; sensitive to submersion
How to listen
Science enthusiasts interested in the detective-style research behind gemstone origins and comparative Earth-Mars geology, as well as opal buyers and collectors wanting to understand authentication and proper care.
0:00–2:03 contains opening pleasantries and ads; jump to 3:03 where the formal interview begins.