What It Says On The Box:
Vermiculite is the most popular product used for incubating reptile eggs. It is able to hold a massive amount of water for its weight and volume and can achieve high humidity over a substantial amount of time. Other advantages are that it doesn’t rot or support microbial growth.
The fine grade is recommended for small, soft shelled eggs and the coarse grade for large hard shelled eggs. Tarantula keepers also often
use Vermiculite with South American spiders like Avicularia and other species which require constant high humidity."