Cabosil,,,fumed silica.... in the production of optical fiber for example, pure silica is vaporized to make fumed silica preforms. These are carefully melted and drawn to make a variety of optical fibers.

When the process doesn't go well, bunches of extremely lightweight fumed silica are left over. Turns out this stuff is incredible thickening stock for epoxy and adds virtually no weight. But adds practically no strength either, in the manner glass or carbon fibers do