They come in search of the usual mineral suspects: emeralds, sapphires, rubies mined all over and transported to a gem fair that is one of the biggest things to happen here each year.
 
They come, too, in search of specimens and oddities, rocks with unusual names and baroque formations: purple apatite from Panasqueira in Portugal; carrolite from the Democratic Republic of Congo; epidote from the famous Green Monster mine in Alaska; sacred Shiva lingams rolled smooth over eons by the Narmada river in India; meteoritic litter like the stuff that rained down on Deputatskoye, Russia, only last week.
 
The umbrella term for an annual trade fair that is a key destination on an international circuit is the Tucson Gem, Mineral and Fossil Showcase. The name scarcely hints at the scope of an event that takes over Tucson each winter — 43 shows in 41 locations, including hotels, motels, dusty parking lots, the city’s big convention center and tent cities set up by frontage road along Interstate 10 — drawing dealers from China, Morocco, Siberia, Tanzania, Australia and India.
 
“There are dealers from literally all over the world,” said Tito Pedrini, a New York-based jeweler who arrived in early February for the American Gem Trade Association, a high-end fair that people in his business scour for exceptional examples of the colored gemstones fashionable now among celebrities and the ultrarich.
 
“You think, ‘Why is the gem show there?’ ” Mr. Pedrini said. “Tucson is such a difficult place to reach.”
 
What started 59 years ago as a show created by 18 local mineral enthusiasts and held in an elementary school has spread over the ensuing decades into a hunting ground for those looking to buy or sell anything from hulking geodes to a gumball-size Tsavorite.
 
Hardly a jeweler at work — from industrial Goliaths like Tiffany & Company and Bulgari to fashionable artisans like Mr. Pedrini, Ted Muehling, Susan Reinstein and Brian Ross of Reinstein Ross, or Ramji Bharany of the century-old Bharany’s in New Delhi — has skipped a visit to Tucson.
The reasons are simple. Gem trends are set there. New strikes are showcased. Peculiarities like the remains of fossilized monkey puzzle tree that the Victorians called Whitby Jet and used to make mourning jewelry turn up again suddenly, ready to be back in style. Freakish deposits appear there, things like a hunk of fluorite the size of a kiddie pool, pried from a mine in England and sold for a six-figure sum before the fair began.
 
It is here that the miraculous and precious stuff teased from the grip of the earth or else winkled out of safe deposit boxes miraculously appears.
 
“New York is one of the centers for colored stones, and it’s very easy to source them here,” Mr. Pedrini said. “I wanted stones that are more rare, and to find those you need to go directly to the source,” he added, referring to dealers in tourmalines or peridots or spinels or opals, precious minerals mined in places as politically and geologically disparate as Australia and Afghanistan.
“The Burma peridots, which people know better, are quieter and more silky,” Mr. Pedrini said of a scarce green stone sometimes mistaken for emerald. “The Afghan peridots are sparkly and lively, more yellowish green.” On his second day here, he came upon a fine 25-carat peridot that even in the midst of war was mined and exported from Afghanistan.
 
“You never know when there might be new finds in one particular area of the world, something different from the standard origins, and that provides renewal” to the jewelry trade, said Matthew L. Hopkins, a Rhode Island-based jeweler specializing in opals. The gems Mr. Hopkins had on offer come from a family-held claim in the Lightning Ridge region of the Australian Outback. In Tucson, renewal, or at least competition, unexpectedly appeared this year in the form of the Ethiopian dealers who have flooded the opal market since deposits of the silicate gem were discovered in the northeastern Wollo province in 2008.
 
“Probably 70 percent of the world’s colored gemstones on the market pass through Tucson during the month of February,” Mr. Hopkins said “Because the dollar is still not doing so well, our market is increasingly the middle class in Asia. The Japanese were really into opals in the ’80s and ’90s.”
 
In that pre-recession era, dealers like Hopkins Opals could charge as much as $10,000 a carat for top-quality black opals. That price plummeted when the fiscally troubled Japanese dropped out of the market; luckily, Mr. Hopkins said, it was then that an emerging Chinese middle class appeared.
 
“The Chinese have not gotten stuck in the realm of ruby, sapphire, diamond,” Mr. Hopkins said. “They have much more room for other stones.”
 
Wandering the aisles of the Tucson Gem and Mineral Show, held in the city’s convention center, Mauro Parietti and Davide Viale, a jeweler and a mineral aficionado from Italy, stopped to look in on Mr. Hopkins’s booth. “You can never find any place else all the things you see here in one place at one time,” said Mr. Viale, who shops for rocks on the basis of their aesthetic value, a practice less common than it might seem.
 
Collectors are as various as rocks, said Thomas Raber, a salesman at Wendel Minerals, dealers based in southern Germany. “Some want only very aesthetic pieces, some want to get only things from certain mines,” he said.
 
“Some look only about rarity and some are systematic and collect only one kind of mineral,” he added, pointing to the prize display in his booth, a grotesque group of naturally occurring “threads” of Saxon silver, found in an old German collection and dauntingly priced at about $58,000 each.
 
Some collect on the basis of emotion because, as Elizabeth Taylor, that incontestable authority on rocks, once told this reporter: “Stones have a life of their own. There’s something mystical about them. They have their own vitality.”
 
It was the vibrancy of a particular Tsavorite that struck Mr. Pedrini, the Manhattan-based jeweler, when he visited Bridges Tsavorites.
 
“I went in the morning, because your eyes are only good once, and so you should only look at gems then,” said Mr. Pedrini, among the rare fashionable jewelers also trained as a gemologist. “And they brought out some stones, which were nice, and then this one that they told me is the most expensive and one they never show.”
 
The discovery of Tsavorite, a form of garnet, is generally credited to Campbell R. Bridges, a Scottish-born, Kenya-based geologist and a legend in the trade. It was in the 1960s that Bridges stumbled upon the rock formations that yielded a stone new to the trade while scouting beryllium for the United Kingdom Atomic Energy Authority. “He was prospecting in Tanzania, and a Cape buffalo charged him,” said his son, Bruce Bridges, 33. “So he dove into a ravine.”
      
Walking to safety, Campbell Bridges (whose 2009 murder by assailants armed with arrows, spears and machetes, in an apparent dispute over mining rights, remains under investigation in Kenya) came upon something flashing green on the ground.
 
“It was unlike anything he had ever seen before,” Bruce Bridges said.
 
Complex politics prevented the strike from being exploited for another decade and delayed the naming of the new gem until 1974. Mr. Bridges’s wife argued for Campbellite (and the slogan “Campbellite by Candlelight”); ultimately Harry Platt, the head of Tiffany, prevailed, and the stone was called Tsavorite after the Tsavo River and the Tsavo National Park.
 
“It’s still a very rare stone,” said Mr. Bridges, who explained that to find a Tsavorite exceeding two carats is vastly less likely than coming upon a comparable emerald. And unlike emeralds, which are sometimes characterized as sleepy and which are by nature delicate, Tsavorite is a tough stone, colored so deeply that it can sometimes seem faked.
 
“The color of the stone I saw was deeply saturated, neon, electric,” Mr. Pedrini said. “This is what happens in Tucson: you fall in love with stones you never knew before.” For now, Mr. Pedrini’s love for that particular Tsavorite must remain platonic: the cost of the 26-carat rock is about a half-million dollars. 
      
“Now,” he said, “I have to find a husband whose wife cannot live without the stone.”