ESPUNGABERA, Mozambique — The last time Mike Phiri was on his hands and knees, clawing through the rough rock of the diamond fields just across the border in Zimbabwe, he dared a soldier to shoot him.

Mr. Phiri and his fellow diamond smugglers had gotten into an argument with the soldier because, for the second consecutive night, he had directed them to a patch of the rich fields where there were no diamonds to be mined. And so they refused to pay him a bribe for allowing them onto the fields, in an area known as Marange. The soldier threatened force.

“If you want to shoot, shoot,” Mr. Phiri recalled barking at the soldier. “But I’m not paying.”

Mr. Phiri and his comrades walked away unharmed, but in more than three months he has not dared return to the fields, an acknowledgment of the increasing dangers for the black-market miners who once dominated an area that may be one of the world’s most lucrative diamond troves.

Ever since President Robert Mugabe of Zimbabwe began to tighten his grip about three years ago on the mines in Marange, where security forces and companies with ties to him are mining at least hundreds of millions of dollars’ worth of stones, the illicit diamond business here has all but dried up. Border towns like this one have become echoes of what they once were.