rockwool

rock +‎ wool

rockwool (countable and uncountable, plural rockwools)

1. Inorganic material resembling fiber, used primarily for insulation and soundproofing.
2. A horticultural growing medium made from natural ingredients, usually including basalt rock and chalk.
   • 1997, George F. Van Patten, ‎Alyssa F. Bust, Gardening Indoors with Cuttings

     In 1985, rockwool was introduced to the United States as an alternative to conventional soil and other soilless mediums. Rockwool is made of thin fibers of volcanic rock or a combination of volcanic rock, limestone, and coke.