Phlogiston (pronounced: /floʊˈdʒɪstʌn/ flo-JIS-tun) was a term that could either refer to the space between the crystal spheres of the Prime Material Plane or to the flammable fluid that permeated it, a rainbow-colored chaotic material that surrounded the spheres. Bizarre creatures inhabited it and preyed on spelljammers traveling between spheres. Spacefarers sometimes called the phlogiston the flow or the Rainbow Ocean.