Rockwool is used as a grow medium - made from basalt & chalk heated to liquid form at very high temperature and spun into fibers, like cotton candy. It's then cut into chunks or granulated. It can be formulated to be hydrophilic or hydrophobic (water-absorbing or water-repellent).
They seemed to hold water just fine. I just prefer sponge plugs. I use the BioDome system from Park Seed. That system has worked really well for me for at least 4 seed starting seasons. In hindsight, it may be that the rock wool plugs I found blowing around in the yard were an anomoly, doesn't make sense that they would not break down in the soil. I probably am not a good source for information since I have limited use. Sorry for the confusion.
I wish that rock wool were manufactured as partially-sintered "stiff pretzles" of fine fibers. The unsintered parts would still hold water, but fairly stiff particles that are shaped like rods or tongue depressers would resist compaction better than spherical grains, limp fibers or blobss of wool.
Preferably helical pretzles with varying ptiches, so they couldn't nest tightly.
Rockwool was first produced in this country in my hometown about 1905 and was a booming industry for many years. At first they dug and quarried the limestone that was nearly on the surface at the west end of town.
Yes, it is nearly indestructible.