A meta material is one where the way molcules are arraigned are what is important. Opal is a natural metamaterial.It's composed of silica spheres 150 to 300 nm in diameter stacked like oranges at a supermarket.
Since this is close to the natural wavelength of visible light it plays with that light giving an opal an almost magical fire.
Man made meta materials promises magic we do not see naturally.
Hence the term opalescent. Interesting!