Don't think the MOHs scale will help with this. I tested some of my various soapstones (1-3 on the scale), catlinite (about a 2-3), and my Black Pearl Soapstone (which is actually a steatite and about 4-5 MOHs) with hydrochloric acid (bottle #1 in my Aquarium Pharm. nitrate test) and they were all inert.Halo said:As a rule of thumb, look up the mineral hardness scale? Just an idea...