What materials are fake coals for gas fireplaces made of?
Originally (back in the old days), they (like gas grills) used volcanic rock aka lava rock. 

Several current vendors use a mixture of rockwool fiber material and vermiculite.  Others list sand and vermiculite.  

Having them on the bottom helps diffuse the gas (by scattering the gas as it rises, so you don't see jets of gas).  The glowing aspects is a nice side benefit.

The newer 'glass' bead gas outdoor fire pits do a similar diffusion and look good when not in use (not so much for vermiculite and rockwool, which look 'ashy' (on purpose)).