Onyx primarily refers to the parallel banded variety of the silicate mineral chalcedony. Agate and onyx are both varieties of layered chalcedony that differ only in the form of the bands: agate has curved bands and onyx has parallel bands. The colors of its bands range from black to almost every color. Commonly, specimens of onyx contain bands of black and/or white. Onyx, as a descriptive term, has also been applied to parallel banded varieties of alabaster, marble, obsidian, and opal, and misleadingly to materials with contorted banding, such as "Cave Onyx" and "Mexican Onyx". Contemporary jewelers have also seized on the stark appearance and vitreous polish luster of onyxes to make them center stones in their own right. Despite onyx’s opacity, gem cutters will often facet this material because the flat surfaces can show a remarkable sheen. Onyxes usually form with very thin layers of color. Although, very rarely, they can occur in nature with layers thick enough to cut a stone as a solid color. Onyxes have straight, nearly parallel bands or layers of color, which allow skilled gem carvers to cut away material to create cameos and intaglios with extraordinary depth and contrast.