The pumice available here, mined in eastern CA and in NV, provides good surfaces for roots and water to adhere to but water and roots do not pass through the particles-- the "holes" in the pumice are locked inside the individual pieces. This material has been a staple for cactus industry for years, but it should be noted that most wholesale production of cacti & succulent utilizes only a mix of organics like SuperSoil and fine perlite. There is often a sharp difference between successful approaches to nursery production versus methods of maintaining plants longer term by hobbyists. Since it is a natural product, what is sold as the same item (pumice) can vary from very clean and sharp-edged particles to soft-edged rounded particles with more dust-like fines included. I agree with Jane that the fines are helpful to the roots, in part by coating them and protecting them with a 'powder coating', especially when plants are dormant. Fines in general, especially organic ones, help with cation exchange (places where nutrients can be stored in the soil). It is unfortunate that pumice is so difficult or expensive for growers to obtain in practical units regardless of where they live. Scoria sounds like what is also known as cinder. This is volcanic material that is crushed in huge quantities to make cinder blocks and this seems to be its nearly exclusive use. A fine grade of cinder (black or red) with pieces 1-2mm is very clean and sharp and one of the best materials I have found to start seedlings of cacti, bulbs, etc. Again, with horticultural demand so low it is no wonder it is difficult to find a source for it. Dylan Hannon