Fulgurites are classified as a variety of the mineraloid lechatelierite. Allophane is an amorphous to poorly crystalline hydrous aluminium silicate clay mineraloid. It is sometimes classified as a mineraloid. Wad can be considered to be the manganese equivalent to the iron mineraloid limonite. Opalescence refers to the optical phenomena displayed by the mineraloid gemstone opal (hydrated silicon dioxide). A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. The color "jet" is a representation of the color of the mineraloid jet. As a mineraloid, no unit cell describes the structure of opal. Unlike many gemstones, jet is not a mineral, but is rather a mineraloid. Opal is another mineraloid because of its non-crystalline nature. Lechatelierite is a mineraloid as it does not have a crystal structure. Other sources call it a mineraloid because it has no crystal structure. A mineraloid is a naturally occurring mineral-like substance that does not demonstrate crystallinity.