It is vermiculite -

I bought a huge bag of it (enough to last me many years) from Home Depot for about $20 - certitified asbestos free (important as some brands at least use to contain asbestos). Asbestos is probably bad for reptile eggs and certainly bad for people to breathe while preparing the incubation medium.

I mixed it 1:1 by weight with water, actually I added a little extra water because the tub is deep so a little extra water is in the lower levels to help keep humidity up.

Some people like to mix a little perlite in with the vermiculite for better air flow but I want to see a valid study and not just anecdotal evidence showing it increases hatch rate before I believe it actually does.
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3.6 L. getula californiae - 16 eggs (Cal. King)
1.1 L. getula nigrita (MBK)
1.0 Pantherophis guttatus guttatus (Corn)
0.1 Pituophis catenifer catenifer (Pacific gopher)
3.3 Elgaria multicarinata multicarinata - 14 eggs (Cal. Alligator Lizard)