rock wool

Noun

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.
   • 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.1997, George F. Van Patten, Alyssa F. Bust, Gardening Indoors with Cuttings