The old building standards used in the past were a bit... less stringent. This meant an assortment of materials was used in its construction. One example is Vermiculite, a natural insulator that's popular because it's cheap & effective.
Vermiculite is an air-tight mineral with a shiny & flaky texture. Once heated to very high temperatures, it transforms into a lightweight, porous material which provides insulation. Still retaining its organic qualities, this organic earth becomes fire resistant. Vermiculite is still used in some projects but is being almost entirely phased out in new buildings. Houses from the 1940s and earlier have a high likelihood of vermiculite usage.
Vermiculite was originally mined in the U.S. from one mine in Montana. This area also produces asbestos, so some of the Vermiculite might be mixed with it. Older homes may feature Vermiculite insulation; if you discover this within your home, please follow these safety guidelines for handling the material: