What is rockwool / mineral wool?

We've been asked this question several times, so we thought it best to make a permanent explanation available...

Rockwool (or mineral wool) is a planting medium that is commonly used in hydroponic growing.  It is manufactured from molten stone, in a process similar to making cotton-candy.  The resulting product has a texture much like fiberglass insulation.  The final product is composed of densely "woven" fibers that form the final shape.  Rockwool is found in many forms, the most popular being "cubes" and "blocks".  In it's final form, rockwool is approximately 97% empty space; it has plenty of space to retain both air and water.

The photograph below shows three rockwool "cubes", at roughly life-size.  Each one has a hole, meant for starting a seed. 

Rockwool is a popular medium for hydroponic planting, as it offers excellent water retention.  It is most frequently used when starting new plants from seed.  The rockwool medium provides the young plant with a safe place to take root, and later allows the seedling to easily be transplanted into a new location.

Rockwool is often used in conjunction with expanded clay planting media.  The rockwool is used for starting the seedling.  Once the seedling is established in the rockwool, the entire bundle (rockwool and seeding) can be planted into expanded clay media for further growth in a hydroponic system.  Rockwool is commonly used in Ebb & Flow, NFT, Drip, and Aeroponics.

It should be noted that rockwool does require pH conditioning prior to use.  The general guideline is to soak the rockwool in water that has been adjusted to pH 5.5 on the day before use.  This will condition the rockwool, and the pH will then remain neutral when it is used for planting.

Rockwool media can be re-used, and can be heat-sanitized between uses.

Generally, rockwool is widely available through hydroponics retailers, and is available quite inexpensively.  It is currently manufactured by dozens of companies, and can be found in a variety of forms.