Talc is a clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent and lubricant; is an ingredient in ceramics, paint, and roofing material; and is a main ingredient in many cosmetics. It occurs as foliated to fibrous masses, and in an exceptionally rare crystal form. It has a perfect basal cleavage and an uneven flat fracture, and it is foliated with a two-dimensional platy form. The Mohs scale of mineral hardness is based on scratch hardness comparison, ranging from 1 to 10, a value of 10 being the hardest of minerals. The hardness of talc, the softest of minerals, defines the value of 1 on the scale. (Any mineral with a value less than 2 can be scratched by a fingernail.) When scraped on a streak plate, talc produces a white streak; though this indicator is of little importance, because most silicate minerals produce a white streak. Talc is translucent to opaque, with colors ranging from whitish grey to green with a vitreous and pearly luster. Talc is not soluble in water, and is slightly soluble in dilute mineral acids.