An astro-miner was a mobile shipyard built by the U'lanna, building starships out of asteroids.
The Stoneship (BC-347) encountered a rogue astro-miner in the Idlewild system, building a Khin'Vagh-class dreadnought out of asteroids. It was destroyed by ramming a K'pak-class corvette. (Star Trek: The Stoneship Files: "A Knight without Armor")
Astro-miners could capture an enemy's schematics. When building starships of a captured design, the astro-miner will build the astro-miner without shields and with a level of comfort that makes it suitable for U'lanna, rather than its level of comfort. (Star Trek: The Stoneship Files: "Generational Ghosts")