The hard seed of the nutmeg tree, nutmeg can be found sold whole or ground into powder. It is best to buy it whole and grate it as needed, so that no flavour is lost. Because it contains a narcotic substance called myristin, nutmeg should only be used in small quantities.
Nutmeg can be replaced with mace, which is the bright red membrane known as "aril" covering the nutmeg seed. It is usually sold ground.