Everyday Uses of Rocks and Minerals - Shelf 5
Garnet is a gemstone composed of metals (calcium, magnesium, iron, aluminum, and/or chromium) bonded to silica. It has a relatively high hardness, harder than silica sand, so small grains are used as an abrasive for both sand blasting and in sand paper.
In contrast, talc, used in baby powder, is a very soft mineral. It is composed of magnesium and silica, bonded with water, so it has some of the same elements as garnet, but the arrangement of its mineral structure makes it very weak -- hence its softness. Soapstone is composed primarily of talc.