Rodger Whitlock wrote, Everybody interested in pumice should be aware that the stuff sold >for landscaping is not necessarily pumice. I have several bags each >of black and red "volcanic cinders"; these are not pumice, but >others might mistake them for pumice because of their volcanic >origin. >Volcanic cinders (for which there is undoubtedly a technical term) >are little blobs of lava that were thrown into the air and hardened >before landing. They are dark, have sharp edges, and are definitely >not "frothy". the technical term for this material is "scoria." Loren Russell, an excellent rock gardener, swears by what is locally called "red rock" (red scoria, loosely red pumice) as a nutrient-rich material, and incorporates the dust and fines from piles of it (which are sold as a mulch) in his planting mixes. Jane McGary Northwestern Oregon, USA