There is a product which used to be available over the internet which was occasionally used as a biomedia. It was called “Growstone”. The name was changed to “Buddystone” in 2019
This is a fused glass product with a size of approximately 3/8 inch. This is recycled glass from the landfill and it absorbs water into its structure. It is 80% void space in a very small open cell rigid reticulated foam structure. This makes it unique. It doesn’t float (after a 24-hour soak) and can be poured into place.
It has an effective surface area of about 40 square feet per cubic foot, making it one of the best ceramic or fused glass products. This is with the caveat that ALL ceramic products are very poor biofilters including Buddystone. It can’t be easily crushed. But it does shed small abrasive particles which will take out pumps.
Note that when one looks at Seachem Matrix media under a microscope and compares it to Growstone, the two products appear to be identical in all their properties. Hmmm is this the “natural stone product” Seachem markets?