For half a century, scholars have searched in vain for the source of the jade that the early civilizations of the Americas prized above all else and fashioned into precious objects of worship, trade and adornment. The searchers found some clues to where the Olmecs and Mayas might have obtained their jadeite, as the precious rock is known. But no lost mines came to light. Now, scientists exploring the wilds of Guatemala say they have found the mother lode a mountainous region roughly the size of Rhode Island strewn with huge jade boulders, other rocky treasures and signs of ancient mining.

It was discovered after a hurricane tore through the landscape and exposed the veins of jade, some of which turned up in stores, arousing the curiosity of scientists.

The find includes large outcroppings of blue jade, the gemstone of the Olmecs, the mysterious people who created the first complex culture in pre-Columbian Mesoamerica, the region that encompasses much of Mexico and Central America. It also includes an ancient mile-high road of stone that runs for miles through the densely forested region.

The deposits rival the world's leading current source of mined jade, in Myanmar, formerly Burma, the experts say. The implications for history, archaeology and anthropology are just starting to emerge.

For one thing, the scientists say, the find suggests that the Olmecs, who flourished on the southern gulf coast of Mexico, exerted wide influence in the Guatemalan highlands as well. All told, they add, the Guatemalan lode was worked for millenniums, as compared with centuries for the Burmese one.

"We were thunderstruck," George E. Harlow, a jade specialist at the American Museum of Natural History, said of the find. "This is the big one." In part, the discovery is a result of the devastating storm that hit Central America in 1998, killing thousands of people and touching off floods and landslides that exposed old veins and washed jade into river beds.

Local prospectors picked up the precious scraps, which found their way into Guatemalan jewelry shops and, eventually, the hands of astonished scientists.

" `Lordy,' I said, `this is Olmec type,' " recalled Russell Seitz, who decades earlier directed a jade hunt in Guatemala for the Peabody Museum at Harvard. "Where did it come from?"

Led by Mr. Seitz and local jade hunters, a team of scientists from the American Museum of Natural History, Rice University and the University of California scoured the forested ravines of the Guatemalan highlands for more than two years.

In the end the scientists made a series of discoveries culminating in bus-size boulders of Olmec blue jade, some astride creeks. "It kept getting better and better," said Virginia B. Sisson, a geologist at Rice University who has recently examined jades in Myanmar as well as Guatemala. The blue jade, she said, "is all over the hillsides."

The exact locations of the outcroppings are not being given, to protect them. Leading archaeologists in Guatemala, though not directly involved, are applauding the finds. Hctor Escobedo of the Universidad del Valle called the jade discovery "one of the most significant" in decades of investigating the Mayan past and said the new deposits probably account for "all of the sources for Mesoamerican jades."

He added that given Guatemala's lack of financial resources, "it is crucial to organize a cooperative effort with international scholars and institutions in order to protect and study the new jewel of our cultural heritage."

Scientists have long known that jadeites, like diamonds, arise deep inside the earth as rocks are cooked at pressures so great that their basic characteristics change. Geologic action over the eons then lifts them to the surface.

The glassy, hard, often translucent rocks occur at only a few known sites around the world. But jade catches the eye because of its astonishing range of colors: white, red, blue, brown, blue-green, emerald green, dark green and blackish. Individual rocks are often mottled with colored specks and streaks.

Early peoples of the Americas considered jade more valuable than gold and silver. The Olmecs, the great sculptors of the pre-Columbian era, carved jades into delicate human forms and scary masks. Mayan kings and other royalty often went to their graves with jade suits, rings and necklaces. The living had their teeth inlaid with the colored gems.

Jade was highly prized in Mesoamerica from at least 1400 B.C. until the Spanish conquest of 1519-21, experts say. But the Spanish were more interested in gold. Soon the skills and lore of jade mining and carving disappeared.

The modern hunt began in the early 1950's. Scientists examining the Burmese deposits found that jade always occurred in association with serpentine, a mottled greenish rock.

In seeking a Mesoamerican source, they focused on Guatemala because much serpentine rock occurred there in the Sierra de las Minas and the adjacent Motagua River valley. A few outcrops of low-quality jade were located near the river. By the mid-1970's scholarly interest was high enough to prompt organized Guatemalan hunts and meetings. The Peabody Museum and the Boston Museum of Fine Arts were among the sponsors, as was Landon T. Clay, a Boston investment banker who collected jade.

But little turned up, and attention shifted to places like Mexico, Costa Rica and Honduras, where archaeologists had found thousands of jade artifacts.

Back in Guatemala, a married couple with a commercial interest in jade kept finding specimens near the Motagua River. The couple, Mary Lou and Jay Ridinger, discovered enough to set up a company, Jades S.A., in Antigua, a quaint tourist town. From a storefront workshop, they sold jade jewelry and Mayan replicas and shared information about some of their finds with scientists.

The turning point came in 1999, shortly after the hurricane swept through. After a long absence from Guatemala, Mr. Seitz, who led the Peabody hunt, was vacationing in Antigua and stopped in one of a half-dozen jade shops that had sprung up.

Curious about what prospectors were finding, he was taken up to the shop's roof to inspect new specimens. His eye fell on a piece of bluish jade roughly the size of a hand. It was highly translucent and unlike anything he had ever seen before in Guatemala. But the shopkeepers could not say where it came from.

Starting in early 2000, Mr. Seitz went back repeatedly to Guatemala as local jade hunters led him higher and higher into the mountains. Miles north of the Motagua, after days of hard climbing, he reached a grassy hillock where the local men had used pickaxes to hack open a huge jade vein.

"It was 2 yards wide and 50 long," Mr. Seitz recalled. "It was blue-green and translucent not grade A, but better than what you get in the valley and better than anything that we saw in the 1970's."

Mr. Seitz returned to the United States with a hundred pounds of rocky samples, and Harvard and the Museum of Natural History confirmed that they were high-quality jadeite. Other scholars joined the hunt.

In 2001, Mr. Seitz returned to Guatemala with Dr. Sisson of Rice University and Karl E. Taube, an anthropologist at the University of California at Riverside, who specializes in Olmec and Mayan artifacts.

Among other things, the team found an ancient dry-stone pathway that wound through the mountains from an old mining area to a habitation and tomb site littered with old clay shards. Miles south of the Motagua, the team found even higher-quality jade: huge boulders of blue. "It's impressive," Dr. Sisson said. "We had worked for a month in Burma and didn't see anything as good as this."

The scientists say the most important implication of their find is the idea that the Olmecs exerted wide influence over the region.

"It suggests that the trade routes were more extensive than we realized," said Dr. Taube of the University of California. "Now the big question is, how did Costa Rica get so much of this jade?"

In December 2001 the scientists quietly published a short article in the journal Antiquity on their discoveries, with Mr. Seitz the lead author. The new deposits, they wrote, are "excellent candidates for an `Olmec blue' jade source."

Dr. Harlow, who organized the recent hunts, now estimates that the Guatemalan jade region, spread across public and private land, is roughly 10 times the area estimated before the hurricane. His team, he said, is keeping maps of the outcroppings vague so as to discourage looters.

Last week at the American Museum of Natural History, he showed a visitor a half-ton or so of Guatemalan jade samples, their blues and greens vivid and remarkably varied.

"This is quite nice," Dr. Harlow said, gazing at a slice he illuminated with a flashlight.

The scientists are applying for grants to pay for sustained research into the region's complex geology and its role in the lives of early Americans, especially the Olmecs.

Charles S. Spencer, curator of Mexican and Central American archaeology at the American Museum, cautioned that the work would take years. For example, he said, the accurate dating of old mines will depend on the discovery and analysis of related artifacts like pottery shards.

The discoverers say they are not especially worried about wide commercial exploitation. So far the Guatemalan finds contain few of the bright translucent green jades highly valued by Asian buyers and markets.

Dr. Harlow said he now wondered why only the Olmecs relished blue jade, when so much was clearly available to the Mayas. He speculated that perhaps changing tastes played a role.

More troubling, he said, is a possibility first raised by Mr. Seitz: that an eruption of one of Guatemala's nearby volcanoes "obliterated the people who knew what was what" and hid some of the blue jades that the 1998 storm exposed.

"There are a lot of good questions," Dr. Harlow said. "The proof will come when we do some of the archaeology."