Talc
Talc (from the Arabic talk) is a silicate mineral, a type of clay. It is the standard for grade 1 in the Mohs scale of mineral hardness. A fingernail can easily scratch it. Talc is composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc and pyrophyllite are very similar, but pyrophyllite may be slightly harder.