Rockwool Grow Cubes by Grodan - DM 4G 42/40
3" X 3" X 2.5" Wrapped. Sheet of 8 cubes.
Cube with hole.Plastic wrapped rockwool cubes guide roots downward. You can set them on top of other rockwool cubes / slabs into which the roots can grow. Rockwool cubes with holes in the top are designed to have the smaller rockwool cubes fit into them. This would be after you see roots start to emerge from the smaller cube.