The box is built from American Black Walnut. American Black Walnut is a hardwood species with a light greyish-brown to purplish-black heartwood and a sapwood that ranges from almost white to a yellow, creamy-brown. The grain is typically straight, but can also be curly or wavy, and the texture is usually coarse, becoming more lustrous with age. It is known for its burrs (burls), stump wood, and crotches, which create a mottled, curly, and wavy figure.