The land the diamond seekers rushed into was not empty. The Bororo people had lived in this stretch of the Brazilian interior for centuries, and it was a Bororo guide who first told João José de Moraes - a miner nicknamed Cajango - about the glittering pebbles at the confluence of the Cassununga and Herons rivers. The Indigenous word for them was toricuiêgo. The Salesian Mission, established in Brazil in 1894, set up a dependency at a place called Merure in the heart of Bororo country, under a French priest named Father John Duroure. The mission's stated purpose was conversion and education, and its presence transformed - and in many ways disrupted - the lives of the Bororo, opening the region to settlement in ways that would change it permanently.

The name Guiratinga is Tupi-Guarani, stitched together from guira (bird) and tinga (white) - the white heron that still stalks the wetlands of Mato Grosso, picking through shallows with careful precision. The town didn't start with that name. It was called Lageado after a local stream, and for years it was simply known as the village of Lageado, belonging to the distant municipality of Santa Rita do Araguaia. After World War II, the federal government decided that two Brazilian towns couldn't share a name, and since a Lageado already existed in Rio Grande do Sul, citizens gathered at the Commercial Association in 1943 and chose Guiratinga - the white bird of the marshes - in its place.

Unusually for a mining town, Guiratinga was planned. Augusto Alves, who arrived with his family in 1920, laid out lots on a grid and required new arrivals to build within its lines. The rush that followed the diamond discovery could have thrown up the usual sprawl of shacks and claim stakes - the bedlam of a boomtown - but Alves insisted on order, and the town that grew on the banks of the Lageadinho stream kept the shape of a deliberate city even as it swelled with Bahians, Mineiros, and Goianos hunting for stones. By 1933 it had been made a city seat; by 1938 it had become its own municipality; and in 1981, Pope John Paul II elevated the local Prelature to a full diocese, with Saint John the Baptist as its cathedral.

The Diocese of Guiratinga covers roughly 45,000 square kilometers of southeastern Mato Grosso - a sweep of cerrado, gallery forest, and the rolling hills that feed the Araguaia basin. From the air, you see the mosaic of a cattle country that grew up alongside the mines: pasture broken by the dark green of riparian forest, red-dirt roads bleeding between fazendas, and the Cassununga still meandering toward the Rio das Garças. The town is 317 kilometers southeast of Cuiabá and 110 from Rondonópolis, linked by the MT-270 highway that connects onward to BR-364, the great road that pushed Brazilian settlement deep into the Amazon and the Pantanal.

The diamonds didn't last. The boom that created Guiratinga also created Santa Rita do Araguaia, Poxoréo, and a string of smaller towns, and most of them shrank once the gravel no longer yielded. Guiratinga stuck - partly because Alves had built it to stay, partly because cattle ranching moved in to fill the space the mines left behind, and partly because the diocese gave it a role in the spiritual geography of the state. The herons are still here. So are the descendants of the Bororo, living now in smaller communities. The river still carries a fine glitter at the bottom of its pools, if you look closely, though the prospectors are mostly gone.