I’ve been thinking about using rock wool as a cheap fibrous reinforcing filler, but it’s difficult to find information about its strength. A study in 01996 by Cáceres, García Hernández, and Rincón tested some rock wool fibers they’d made in their laboratory from Canary Islands basalt, using what seems to have been a sort of 10,000-rpm graphite cotton-candy machine they didn’t include a drawing of, getting 639 and 717 MPa, with Young’s modulus of 89 and 86 gigapascals.
I don’t know if this is comparable to the strength of commercial rockwool, but it’s a few times higher than mild steel (A36 is 250 MPa) and a few times lower than Spectra (2500-3500), basalt fiber (4840), or S-glass (4710), and much higher than polypropylene (12-43), PMMA (72), or HDPE (26-33). They remark that it’s lower than other people report for basalt fibers, but point out that frictions among the fibers in the wool would be expected to provoke surface defects and breakage.
The Young’s modulus they report is higher than concrete (30 GPa), bone (14), wood (9-12), plastics (0.228-3.5), magnesium alloy (45.2) or pure aluminum (68); similar to tooth enamel (83), Kevlar (70-112), carborundum (90-137), or stinging nettle fiber (87); and lower than brass (106), bronze (112), copper (110), titanium (116), or A36 steel (200).
A few different papers report that incorporating rock wool into plastics weakens them; for example, Aykanat and Ermeydan, which incorrectly claims that PLA biodegrades in 2-4 weeks, and didn’t use any coupling agents. Also they seem to have gotten a lot of porosity in their PLA.