Little check...Seramis is just clayballs. Can be bought for less than 10$ for a 25 liter bag. I used it for the first time this year. I mix it with CCM Mediterranean soil. Don't know what is better though: perlite or clayballs..
The clay balls are expanded clay during a heat process, which is different form the natural stone.
I use a organic potting mix, which has coarse pet moss and Montmorillonite clay 2-10 mm.
These are not round, I will take a picture. The Montmorillonite clay swells with the addition of water, then it contracts as the water evaporates or is taken by the plant, so it results in more air pockets.
I also add perlite to the mix to make it lighter, as I feel like the soil gets too much water. Perlite is good for the winter but very bad for summer ( more watering, 2 times a day)