Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas.

For gravity settling, this means that the particles will tend to fall to the bottom of the vessel, forming a slurry at the vessel base.

If this account is accurate, it would rule out the Black Stone being an agate, basalt lava or stony meteorite, though it would be compatible with it being glass or pumice.