Grodan Rockwool Cubes (1.5 Inches) 45 Per Pack by Grodon. $12.75. Provides 20% air space when saturated. Does not contain nutrients. An inorganic and sterile growing medium made from spun molten rock. A superior medium for rooting cuttings. Condition by soaking for eight hours. Three strips of 15 wrapped Mini-Blocks - 45 total blocks each measuring 1.5 in. square.