Hello Del, Thanks for this information. In France this is called "pouzzolane" and comes in various sizes 0-5, 5-10, 10-20 mm. I am using the 0-5 mm grade for mixing with compost to get our pottingsoil. The 5-10mm material is used as a mulching layer (2-3cm thick=1 ") Greetings from south of France http://www.bulbargence.com/ -----Original Message----- Pumice is volcanic rock which is full of gas bubbles and is so light that it will float on the ocean.Basalt is also from volcano's,but is a very heavy, dense rock,made below the earths surface.Basalt is often crushed,for its multiple minerals.