SuperHatch incubation medium is calcined clay. It's almost the same as Seramis. I can't recall right now what Seramis is made of, but it's almost identical. I've been using Profile Aquatic Pond Soil (a.k.a. Turface) for 12 years now. It's much cheaper if you buy it at garden centres or wherever pond supplies are sold.
All three products are fired clay, some variation in formula and manufacture, but it's all the same for our purposes. The water is held in microscopic pores in the particles, so you won't see any standing water like you would with perlite, and no soggy mess like vermiculite. It doesn't pack down tight, allows air transfer around the whole egg, yet still keeps an evenly moist environment.
The clay also seems to hold heat better, which makes any temperature change a slow process, no shock to the egg with sudden heat loss.