Apache Tear (foam)

Sounds like the mineral that is used industrially to make the
insulation beads known as “Perlite”. From what I remember from the
spec sheets of a decade ago, this was a ground up mineral of the
obsidian type which (and I know it sounds unlikely) contains
sufficient water to foam the material when heated to high temps. In
the case of Perlite the small grains, about 0.2mm diameter, were blown
through an air/natural gas flame, and came out as beads 1 to 2 mm

Kevin  (NW England,  UK)