Jean Railala plunges his rectangular sieve into the chocolate-colored river.
Bending his knees, the 23-year-old hauls out a tray full of grayish brown gravel and starts meticulously sifting through it with his hands. This could be the lucky one.
Pebbles, rocks, bits of silt and gravel are cast aside as he runs his mud-caked fingers over the wire mesh. Suddenly, he spots a flicker of light.
With his thumb and forefinger he plucks out a tiny crystal and holds it up to the sun, squinting. A sapphire?
"Ah, no. Quartz," he says, throwing it into a nearby bush in his disappointment.
Railala is one of hundreds of Madagascans who set up makeshift camps at Manubo Vaovao, scene of the huge Indian Ocean island's latest sapphire rush.
The clear, dark blue precious stones were discovered at the site on an empty patch of savanna this year. Locals say that a small shantytown of fortune seekers sprang up around it almost overnight.
Grass huts and tents improvised from plastic sheets sprawl through the settlement, where young children play in the dirt and women grill cassava along the roadside.
"I used to be a farmer in a small village," says Railala, who brought his wife and two children with him to Manubo Vaovao. "Now we want some money."
Madagascar has long been a lucrative source of precious stones for gem traders.
Dealers from New York, Paris, Bangkok and Sri Lanka's Colombo come to the country to buy sapphires, emeralds, rubies and other jewels found in the earth or in rivers throughout the island. The gems are sold to make brooches, rings, bracelets and other jewelry.
"This is one of the best places in the world to buy sapphires," said Seyed Faluldeen Moulana, a Sri Lankan who owns a shop that trades in rough stones in gem-rich southwestern Madagascar. "They're not always the highest grade, but there are many."
Few Madagascans benefit from the trade. The government says most stones are smuggled out. Even when traded legitimately, profit ends up in the pockets of foreigners with the technical know-how and experience to sell them for a high price, officials said.
Most poor Madagascans work as low-paid, unskilled labor, digging in the mines.
Aiming to address this imbalance, Madagascar opened a school in October offering a training course in the science of precious stones.
The school teaches small-time Madagascan miners and traders how to find, recognize and value gemstones. Another course teaches how to cut precious stones into jewelry, more than doubling their value.
"Up until now, most gems get shipped offshore while they're just pebbles," said Tom Cushman, a U.S. gem trader who set up the project. "Then they get cut and sold in, say, Thailand. And most of the profit stays in Thailand."
The gem school is part of a $32 million World Bank project to develop Madagascar's mineral resources, most of the rest going to geologic mapping to build a better picture of the deposits on the world's fourth-largest island.
Though Madagascar is largely unexplored, mining experts think it has big reserves of nickel, sapphires, rubies, diamonds, aquamarine, emeralds, gold and bauxite.
"Geologic mapping is the most expensive part," said Cushman. "We've got planes flying over the entire country, which is enormous, with sensors to try and build up an idea of what there is."
For the government, keen to find new sources of investment on the island of 17 million people, three-quarters of whom live on less than $1 a day, mining is a key sector.
The World Bank thinks it could bring $400 million a year to Madagascar, one of the world's poorest countries.
The mining of some minerals requires a large industrial effort, but experts say gemstones are well suited to small operations because they can be extracted with just a shovel and a sieve.
"The greatest need is to train the small traders," said Nadine Ranorosoa, coordinator of the government's mineral resources project.
"Education is first, so individuals can get the necessary skills. Then later we can look at mechanized mining techniques."
Mechanization is still a long way off for gem traders such as 43-year-old Jeanuel Andianasulu, who has just opened a new sapphire mine near the trading town of Ilakaka.
His 10 hired men dug 100 feet down into the sandy earth for a year before reaching the gravel layer, where sieving for sapphire crystals can now begin.
"Until we sieve, we can't be sure of what we'll get," he said. "But we did research, and I hope there will be top quality sapphires."
Andianasulu said about 100 tons of gravel must be extracted. From that, he expects to get about 2 pounds of sapphires.
"It should be just enough to make a profit," he said. "But the hard part is to sell them."