LA TRINIDAD, Mexico — Héctor Montes has been around opals his entire life and has held a concession from the Mexican government to mine the stone for 40 years. But at 76, he said he could still feel a rush of adrenaline when he picked up a raw opal that had an especially promising glint — he never knows what it will look like in its finished state.

“There are no two alike,” he said of the stones he shapes and polishes. His workshop, strewn with rocks and lapidary equipment, is part of the family opal business that he runs in this community of about 2,500 residents in the central Mexican state of Querétaro, one of the two main regions in the country where opal is mined today.

“I get tired here, but I don’t get bored because of the variety of colors that we have,” Mr. Montes said as he polished an opal using finer and finer grits of sandpaper. “You’re always eager to see how each stone will turn out.”

He pointed to some of the stones he had at hand: fire opals, rainbow opals, peacock opals, among others. The more colors an opal flashes and the more intense those colors are, the higher the quality, Mr. Montes explained.

Opals are not the world’s most valuable stone, he said, because they lack the hardness of some other gems, such as diamonds. “But in beauty,” he added, “there is no stone that can be compared to it.”

Mr. Montes caught the “fever,” as he called his passion for opal, early on. When he was 5, he would spend time in a nearby jewelry workshop looking through the floor sweepings for any bits of stone that he could sell for a few pesos to spend on candy. By the time he was 12, he was developing his lapidary skills — cutting, shaping and polishing stones.

His grandfather and three uncles were miners, and Mr. Montes inherited a concession to mine a 20-hectare (roughly 50-acre) tract on a hill called El Redentor (the Redeemer). The concession has been in his family since 1894.

Thin veins of opal — Mr. Montes called them “threads” — were deposited here over time as water seeped into the cracks of volcanic rock called rhyolite. Miners in the area excavate small open-pit mines to get at the deposits.

In Mr. Montes’s operation, called Minas de Ópalo El Redentor, crews use small amounts of dynamite and then an air compressor drill to dislodge sections of rhyolite, taking care not to damage the opal.

After that, the process is slow and artisanal, as workers use chisels and sledgehammers to extract chunks of rock and then break them apart, one by one, to see what is inside. Rocks that do not contain significant amounts of visible opal are tossed into a wheelbarrow and hauled to the surface with a pulley.

Mining, Mr. Montes said, is an unpredictable venture that takes persistence, perseverance and a willingness to pick oneself up and start over after failure.

“You can find the fortune that you could never even in your dreams imagine having, but you can also spend your life dreaming that you will find it and never manage to,” he said.

In addition to extracting opal, the family business offers tours to people who want to visit the mine and try their hand at finding a gemstone in the ever-growing mountain of rubble. Visitors are provided with hammers and chisels and can keep any stones they find.

“To work a mine, you need another mine” — in other words, another source of income, said Mr. Montes’s son, Fernando, who heads the tourism operation. Mining by itself is hit or miss, he explained; for every 10 tons of rock removed, the business might recover one kilogram (2.2 pounds) of precious fire opals.

Then there are the rare times when a crew stumbles on a large deposit. The last time it happened at El Redentor was in 2016, Fernando said. And, “with that bonanza, I was able to build my house.”

As Rufina Ugalde, Mr. Montes’s wife and Fernando’s mother, who is involved in the business and manages a small opal shop, said: “There’s nothing certain in mining. It’s like the lottery.”

Small But Distinctive

Mexico is a relatively minor opal producer, lagging far behind Australia and Ethiopia. At Opal Auctions, which bills itself as the world’s largest online marketplace for opals, Mexican opals accounted for about 4 percent of the loose opals sold through the site in 2022, according to Ross Sedawie, the company’s head of business operations. (About 72 percent of the opals sold were from Australia and 20 percent from Ethiopia, he said.)

But Mexican opals are always in demand because of their bright, fiery colors, according to Joel E. Arem, a veteran gemologist who has a Ph.D. in mineralogy and has written extensively on gemstones. “You get a really kick-ass, killer Mexican opal, and it is glorious,” he said.

Opal, like quartz, is composed of silica, a compound that makes up much of the Earth’s crust. In the case of quartz, Dr. Arem explained, the silica crystallizes and forms angular structures; opal, by contrast, is amorphous, a collection of microscopic spheres. Carried by water, these spheres, which are gelatinous in consistency, work their way into tiny spaces within rocks, where they settle and solidify. This process happens all over the planet and produces what is known as common opal, Dr. Arem said.

Precious opal, the kind most prized for jewelry, is rare. It occurs when the microspheres are uniform in size and the conditions are just right for them to stack into perfectly aligned layers (like a pile of ball bearings, as Dr. Arem described it). The arrangement allows the light to bounce and reflect as it moves among the stacked spheres, producing a prismlike effect that jewelers and gemologists call fire, or play of color — the quality that makes an opal precious.

Dr. Arem noted that the term for the effect should not be confused with the term fire opal, which describes any opal with a body color in a palette of yellows, oranges and reds, indicating the presence of iron.

Mexico is best known internationally for its precious fire opals — fire opals with vivid play of color — but even here, these are few and far between. Fernando Montes said that for every kilogram of opals that his family’s mining operation unearths, there may be just one gram (.035 ounces, or 5 carats) of fire opals.

Nathan Renfro, senior manager of the Gem Identification Department at the nonprofit Gemological Institute of America, said that stones known in the trade as fire opals came from several places. He has even unearthed some himself in California. But, he said, Mexican opals stand out.

“If you have the combination of the red to orange body color with the play of color, there’s probably no other deposit that’s better to produce that material,” he said in a video interview.

Mr. Renfro said that over the years, he had also seen some “novelty” types of opal from Mexico, including a purple variety with no play of color, a blue-bodied opal with play of color and a spotted stone called leopard opal.

For a fee, consumers can submit gems to the institute’s laboratories for analysis to identify the type of stone and assess its quality. Scientific instruments are used to measure the physical properties of a stone to determine whether it might be, say, a precious opal.

“If we see the play of color, that’s almost a slam dunk,” Mr. Renfro said. “We know it’s opal because there’s really not many materials that have such a phenomenon.”

But there is at least one exception: synthetic opal. Mr. Renfro stressed that consumers should buy stones only from reputable dealers who stand by their products and clearly identify any synthetic materials.

Another potential problem: Because opals contain water molecules in their structure, they can dehydrate and develop superficial cracks, a process called crazing. Gem dealers or sellers may put oil or resin in the cracks to hide them and, in that case, an analysis by the gemological institute would identify that as a treated stone.

Although consumers have no real way to know whether a specific opal will end up crazing, Mr. Renfro said, they can help protect their stones by not exposing them to extreme temperatures.

He said that once, at a trade show, he saw a beautiful Ethiopian opal ring on display under bright lights and noticed it had a crack that the designer said had not been there before. Even if the lights weren’t the cause of the crack, Mr. Renfro noted, they “were not doing the stone any favors.”

Jewels on Fire

In the tourist mecca of San Miguel de Allende, Mexican opals can be found for sale in a range of colors and styles, from classic cabochons to irregular, free-form stones in contemporary settings.

Some of the clearer stones, often referred to as crystal opals, are faceted like rubies; in other cases, an opal may be left in the matrix (the stone where it is embedded), which becomes part of the finished piece.

Opals also may be mounted on a dark backing material, such as onyx, to intensify their colors. These combinations, called doublets, are typically less expensive than their solid-opal counterparts.

At a jewelry store called Linaje .925 Fine Jewelry in San Miguel, customers often will ask for fire opals, according to Jazmín Carreón, who owns the shop with her husband, Constantino Gómez. She said she explained to them that the iridescence that characterized opals was not confined to one variety.

“All of them have fire, because all of them have that play of color,” she said.

Mr. Gómez, who designs most of the pieces sold in the store, said he was drawn to opal because it is native to the region and because the “infinity of colors” opens up so many creative possibilities. He works with lapidaries and jewelers to craft pieces that will set off each gem.

“The stone is always going to be what guides us,” he said. The pieces that are hardest to sell, according to the couple, are earrings because the nature of opal makes it hard to match them exactly.

Opals, just like people, are one of a kind and come in all shapes, sizes and colors, said Fabiola Salgado, who manages a store in San Miguel called The Opal Mine, which has walls lined with rhyolite rocks and display cases made of suspended wheelbarrows. Because each stone is different, Ms. Salgado said, she knows that when she puts on a necklace, “nobody else is wearing my necklace.”

Prices for opal jewelry vary widely. A gold ring with an opal at the Opal Mine, for example, can cost $500 to $7,500, depending on the kind of gold used and the quality of the stone, Ms. Salgado said.

The Opal Mine’s headquarters (and three other stores) are in Puerto Vallarta, which is in Jalisco state, the other main source of Mexican opals.

“We say at our stores that Jalisco has the best of Mexico: mariachis, tequila and opals,” the Opal Mine’s owner, Gregorio Brito, said in a video interview. In addition to its stores, the company has two manufacturing locations and has developed partnerships with a few opal mines in Jalisco, Mr. Brito said.

Several factors have caused periodic slowdowns in local mining in recent years, he said, including security concerns related to drug cartel activity in the area. And tequila’s growing popularity has led many people to plant agave, the plant tequila is made from, instead of working in the opal mines.

Both Mr. Brito and Héctor Montes, at the opal mine in Querétaro, said that historically, foreign dealers — from Japan, Germany and other countries — have been the main market for Mexican opals.

Mr. Brito said he would like to see the stone have a higher profile in Mexico and become a source of the kind of national pride that people now feel toward tequila.

“These are stones that have life,” he said, adding that, to him, the vibrant colors found in opals represent the liveliness of Mexican folklore and culture.

Mr. Montes had made a similar point a few days earlier. “Here in Mexico, we have opals in all the colors we could imagine,” he said. “We are a country of privilege that hasn’t known how to value what we have. It’s that simple.”