the skinny on rockwool:
Rockwool is a horticultural growing media made from the natural ingredients Basalt rock and Chalk. These are then melted at 1600° C into a lava which is blown into a large spinning chamber, which pulls the lava into fibers like "cotton candy." If you have ever visited a volcano you have probably seen these fibers flying around in the air surrounding the volcano. Once the fibers are spun they are then compressed into a mat which is then cut into slabs and cubes. The rockwool granulates are just bales of uncompressed fibers. The process is very efficient, producing 37 cubic foot of wool from 1 cubic foot of rocks. Since rockwool is born in fire it renders the product chemically and biologically inert and creates the ideal growing medium for hydroponics. Since its development in Denmark in the early 1970's, rockwool has become the major vegetable and flower production medium throughout Europe and North America.
Horticultural Rockwool growing media is primarily available in two general formats. First, as rigid slabs, blocks, and cubes known as "bonded" products because the fibers are held together with a "gluing" or binding agent which renders them stiff and brittle. This is the primary format for the vegetable and cut flower industries. Secondly, rockwool is available as a highly refined and consistent hydrophilic or hydrophobic granulate which is basically water absorbent or water repellent. This format can be used as a component in various peat moss based soilless media or for ground bed incorporation to improve the tilth of heavy clay or light sandy soils.
The formed products are available in various sizes and shapes which are adaptable to many applications. We offer various sizes of seeding and propagation cubes and plugs, blocks and slabs. The most important characteristic of all formed rockwool lies in the fact it allows growers to quickly respond to fluctuations in the plant's rooting environment. The rockwool being an inert media means that rockwool fibers do not modify or restrict the availability of nutrient to the plants. Due to this zero Cation Exchange Capacity (CEC), the material can be leached of all fertilizer "salts." In addition to this, rockwool possesses a near-zero absorption capacity for water, thus allowing more water to be available to the plants when compared to production in soilless media. Of the total amount of nutrient solution applied to a rockwool slab, only 2% is unavailable for plant uptake. Organic media such as peat moss and sawdust, possess a 65% water holding capacity and nearly 4-8% of the nutrient solution is absorbed to the colloidal structure of the material and hence is unavailable to the plants. Due to the unique pore structure of the rockwool, it can safely receive large volumes of nutrient solution without leaving it water logged. After excessive soaking, the slab, block or cube will drain sufficiently, in a short period of time, so that 40-50% of the pore space is occupied by air. Deliberate leaching of the slab with excessive nutrient solution can be used as a management tool to control crop health and maintain optimum growing conditions in the root zone.
taken from: http://www.hydroponics.net/learn/rockwool.asp
It [rockwool] is inhert and so will not pollute the aquarium or affect the pH or water quality when the plants are put in the water.
Most countries do not allow soil or other organic growing materials to be brought in through their airports and because cultiwool is inhert and not organic there are no problems with phytosanitary requirements.
Cultiwool has excellent growing media properties like a high air fill porosity, it wets easily and has a high nutrient exchange capacity
Cultiwool is also a clean and easy medium to work with so helps with planting, cleaning and packaging and transport of plants.
taken from: http://www.aquariumplants.co.za/media.htm
There are a ton of great reasons rockwool is superiorto soil-based substrates in horticultural applications, most notably aeroponic, hydroponic (i.e. immersed) growing operations. Those situations in which rockwool is to be used long-term, fully submerged pose different problems and make its use is less desirable, hence the reason most informed hobbyists remove it before placing the plant in the aquarium; and it's unsightly...