The Outdoor Plus OPT-RF37EKIT-GRY-NG 37" Gray Natural Gas IID Sienna GFRC Concrete Fire Pit. Sienna Fire Pit, an earth shade that represents our environment. Its hemispherical structure accentuates the beauty of itself. Our Fire Pits are individually handmade and proudly manufactured in the U.S.A. The fire pits we manufacture are durable and long-lasting in a variety of environments. Because of its handmade nature slight variations in color and texture may occur.