An absorbent powder and thickening agent.

An interesting read regarding silica and its ability to penetrate your skin:

In vivo penetration of bare and lipid-coated silica nanoparticles across the human stratum corneum. Colloids Surf B Biointerfaces. 2014 Oct 1;122:653-61. doi: 10.1016/j.colsurfb.2014.07.046. Epub 2014 Aug 12.

Diatomaceous earth is mostly silica (80-90%) and is used as an Abrasive, Absorbent, Opacifying, Anticaking agent. The rest of the components of this ingredient are 2 to 4% alumina (attributed mostly to clay minerals) and 0.5 to 2% iron oxide.