All stone wools are not the same. The best are produced from pure basaltic rock (diabase). The stone wools produced from diabase have a mineral balance that is inert and nonreactive. A few stone wools are produced from slag left over from smelting operations. These stone wools contain a high proportion of metals and may be somewhat reactive with the nutrient solution. High quality stone wools 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 stone wool should have is uniform wetting. The stone wool should wet easily but not remain water-soaked. Good drainage is important. Ease of handling (labor savings), better control over nutrition and better disease management are all reasons for this trend. These factors all point to more yield at lower cost.
