sounds like perlite.
Don't buy any dirt that comes in a bag, waste of time and money.Any suitable monitor enclosure will require more than a few bags of dirt.
Take your truck to the landscape material yard and find suitable dirt/sand. Every state is different but they sell fill dirt and sand everywhere.When it comes to substrate, if it comes in a bag, or has a picture of a reptile on it, don't buy it!
Ben