Every year, millions of migratory birds flock to Arctic Alaska. Hundreds of thousands of caribou use the tundra, rich in plant life, as their calving grounds. Alaska’s North Slope is also rich in other natural resources: oil, gas, minerals. But one important thing is lacking: Rocks. “Yes, gravel is a precious commodity on the North Slope,” said Jeff Currey, an engineer with the state’s Department of Transportation and Public Facilities who works in the agency’s Northern Region Materials Section. For decades, Currey said, the state has been searching for gravel all over the North Slope, with limited success.