Slate is commonly gray in color, although you may find green, yellow and red hues. It is a stone which has been metamorphosed from shale-that is, it consists of clay-like materials that have undergone change under heat and pressure. For the layman, slate can be recognized by its sheet-like structure. The material is usually thin and if broken in half, will flake off into sheets. Slate is seldom highly reflective unless a coating is placed on it, and its surface is usually uneven unless machine- sanded.