Talc or talcum is a clay mineral composed of hydrated magnesium silicate with the chemical formula H2Mg3(SiO3)4 or Mg3Si4O10(OH)2. In loose form, it was one of the most widely used substances known as baby powder, along with corn starch. It occurs as foliated to fibrous masses, and in an exceptionally rare crystal form. It has a perfect basal cleavage, and the folia are not elastic, although slightly flexible.
Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 1 as the hardness of talc. As such, talc can easily be scratched by a fingernail. Talc has a specific gravity of 2.5–2.8, a clear or dusty luster, and is translucent to opaque. Talc is not soluble in water, but is slightly soluble in dilute mineral acids. Its colour ranges from white to grey or green and it has a distinctly greasy feel. Its streak is white.
Soapstone is a metamorphic rock composed predominantly of talc.