Rockwool is a man-made mineral fiber. The vast majority of rockwool used in the world is used for insulation purposes much like fiberglass. However, the properties of rockwool can be substantially changed by adjusting the mineral content. In the early 1960s, it was found that following several modifications to the manufacturing process rockwool would support and, under the right handling practices, promote plant growth. This specially produced horticultural rockwool is what is primarily sold as a hydroponic substrate. (From now on the term “rockwool” will specifically refer to the horticultural grade product).
Rockwool is manufactured by melting basaltic rock and spinning the melt into fibers. Immediately following spinning, a binder is added to the fibers and they are compressed and cured into large slabs. By adjusting the amount of pressure, the density of the media is adjusted. The large slabs can be cut into smaller slabs and propagation blocks for easy handling. The spun fibers are also formed into a granulated (flocked) product which can be handled in a manner similar to bales of peat.
All rockwools are not the same. The best are produced from pure basaltic rock (diabase). The rockwools produced from diabase have a mineral balance that is inert and nonreactive. A few rockwools are produced from slag left over from smelting operations. These rockwools contain a high proportion of metals and may be somewhat reactive with the nutrient solution. High-quality rockwools should have a uniform fiber diameter, even binder distribution and a low proportion of shot (mineral pellets that have not been spun into fibers). The most important quality a high-grade rockwool should have is uniform wetting. The rockwool should wet easily but not remain water-soaked. Good drainage is important.
The wetting characteristics of rockwools vary considerably. Rockwool fibers are naturally hydrophobic (they repel water) because of the presence of mineral oil. In the highest quality rockwools the mineral oils are removed during the manufacturing process and mineral wetting agents are incorporated in the melt. This form of horticultural grade rockwool naturally attracts water and wets easily. While this is the highest quality process the actual quality of the rockwool will depend upon the care taken in manufacturing. Other manufacturers simply add or provide a chemical surfactant (essentially a refined soap) that allows the naturally hydrophobic rockwool to hold water. The major drawback to this approach is that the wetting agent must be supplied regularly or it may wash out.
The best way to determine the quality of the various rockwools available is to test them. See if the wetting is uniform or if there is a large proportion of shot. If you want to try a new product it is worth the time to test it.