Onyx is a type of cryptocrystalline quartz, composed of alternating layers of chalcedony and moganite. It typically forms in the voids of lava rock or other compacted materials. The distinctive black color in black onyx is primarily due to the presence of minute impurities, mainly iron and organic matter, which result in its opaque appearance. Onyx can also occur in various colors, including white, gray, brown, and banded variations. These color variations are influenced by different impurities and the specific geological conditions in which the onyx forms. As a natural gemstone, onyx is valued for its unique layering and bands, making it a popular choice for jewelry, carvings, and decorative items.