In its purest form, sandstone is glassy and white in color, but the material is more typically found in shades of brown, yellow, red, and gray. The color of the rock is determined by the type of minerals it contains, as well as the characteristics of the material that bonds them together. The presence of iron oxide, for instance, results in a yellow, orange, or reddish-brown hue, while significant amounts of potassium feldspar impart a pinkish tint to the rock. The occurrence of gray sandstone is typically a sign that that rock contains calcite cement.