Rockwool cubes come in many different sizes. They all have holes in them to allow for starter cubes to be placed in them. They are sold by the strip or by the case. The 4" x 4" x 4" and the 6" x 6" x 6" are the most popular for growing plants to full term. The smaller blocks are generally used as a supplemental growing medium (placed in grow rocks, on a slab or directly into a hydroponic system.)
Stonewool products are made from basalt rocks and chalk, which come from the Earth. The Grodan Stonewool manufacturer heats rocks to 1600° C, to create lava. The lava is blown into a spinning chamber, which makes fibers similar to cotton candy. The fibers are packed into mats, from which Grodan cubes, blocks and slabs are cut. Small tufts of fibers are also produced and bagged as "Stonewool granulate".
Grodan stonewool was developed in Denmark in the early 1970's. The Grodan stonewool product is the main reason that most of the worlds greenhouse vegetables are now grown hydroponically. Grodan stonewool takes up 80-90% of it's own volume. It is wrapped in biodegradable plastic and the stonewool can be re-used in your garden to improve the aeration and waterholding capacity. Grodan stonewool has the proud distinction of going into space on Challenger in 1986 and is still being used in NASA's testing with hydroponics.