Some minerals are nacreous or pearlescent, giving the skin a shining or sparking appearance. The luster ranges from vitreous to nacreous and silky (if fibrous). Iridescence is also seen in the much rarer polar stratospheric clouds, also called nacreous clouds. Its interior is nacreous and white or yellowish in colour. The Fantail goldfish may have either metallic or nacreous scales and normal or telescope eyes. Celestials with nacreous scales are known but rarely seen. It is sometimes called "nacreous sulfur" or "mother of pearl sulfur" because of its appearance. Scalation may either be metallic, nacreous (calico) or matte. Their shells lack a nacreous layer, and the gills are lamellibranch in form. They may either have metallic, matted, or nacreous scales. Those that show mother-of-pearl colors are given the name "nacreous". Conversely, the nacreous layer is derived from the main surface of the mantle. Veiltails are available in many colors and may have either metallic or nacreous scales. The nacreous layer of monoplacophoran shells appears to have undergone some modification. They have typical primitive characters like the nacreous interior of the shell.