MUMBA, Democratic Republic of Congo - The hillside bears a gouge like a wound, and as the sun sets, the miners put down their shovels and climb the crumbling walls of their trenches by the hundreds.  The fortunate ones clutch tiny plastic bags of black sand, a pound of which counts as a windfall.  Twice a week a man called Pierre will come with his soldiers and pay as much as $10 for it.
No one working in the mine is quite sure why.
"We don't know the importance of col-tan," said Martin Nkibatereza, leaning on his shovel.  "I mean, how is it useful?"
Col-tan is short for columbite-tantelite, an ore rich in the element tantalum, as well as niobium, another high-tech metal.  The ore is nothing less than the wonder mineral of the moment.  In processed form, col-tan is vital to the manufacture of advanced mobile phones, jet engines, air bags, night-vision goggles, fiber optics and, most of all, capacitors, the components that maintain and electric charge in a computer chip. 
This past Christmas, when shoppers fumed at the shortage of PlayStation 2 platforms on which to play war games, the reason was a global shortage of the black sand that Nkibatereza leaves his hut at dawn to collect - provided the shooting from a real war has not forced him to spend the night hiding in the bushes.
Col-tan, is an ingredient in that war as well.
The two wars that have ravaged Congo for four of the last five years have been funded by minerals, a substantial portion of which are siphoned off for leaders of the armies fighting them.  And in Congo's eastern section, home to some of the richest col-tan deposits in the world, the mineral in recent months achieved a prominence commensurate with its value, which spiked spectacularly in the closing months of 2000. 
The first time the world price doubled, Congolese peasants who had been mining gold were instructed to forsake it for col-tan.  When the price doubled again - and then again and again, to more $200 a pound - the rebel group that controls the are declared a monopoly on exports.
"I mean, we are at war," said Adolphe Onusumba, president of the Rally for Congolese Democracy, the rebel group sponsored by Rwanda, whose own war effort is also funded by col-tan.  "We need to maintain the soldiers.  We need to pay for services."
As much as any of Congo's fabled mineral riches, and lately, far more than most, col-tan explains what all those armies are doing in Congo.  Pursuit of any one commodity may not explain why six foreign countries, two rebel groups, and assorted militias came here to fight.
When the Rally rebels and their Rwandan backers started the current war in August 1998, Congo's wealth of gold, diamonds, and copper was well known, but almost no one had heard of col-tan, then selling for less than $20 a pound.
But with the price of a pound of col-tan exceeding $100, or $200,000 a ton, the unit by which it is exported by chartered cargo plane to Europe - the trade goes a considerable way toward illustrating why the belligerents have been so reluctant to depart.
"At least around here, people look for col-tan more than for the gold and the diamonds," said Bizima Karaha, the Rally interior minister."

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