Slate is a dense, tough, durable rock which is mined from quarries generally from the eastern United States. Slate, which is practically non-absorbent, was formed over 500 million years ago. Roofing slate comes in many different sizes, thickness, and colors. The basic colors of roofing slate are black, gray, green, purple, and red. Exposure to weather causes all slate to change slightly in color. The degree of color change varies with different slates. Those slates which exhibit minimal color change are called “unfading” slates. Those slates which exhibit a more obvious color change are known as “semi-weathering” or “weathering” slates. Roofing slate thickness are a standard 3/16 of an inch thick and can be up to 2 inches thick.The longevity of a slate roof can be any where from 50 years to 400 years depending on the quality of slate used.