A mineraloid is a mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that lie outside the generally accepted ranges for specific minerals. For example, obsidian is a glass and not a crystal. Jet is a dense form of coal.

By definition, a mineral has to have an ordered atomic arrangement, or crystalline structure. There are some Earth materials that fit all other parts of the definition of a mineral, yet do not have a crystalline structure. Such compounds are termed amorphous (without form).

Some of these amorphous compounds are called mineraloids. These usually form at low temperatures and pressures during the process of chemical weathering and form mammillary, botryoidal, and stalactitic masses with widely varying chemical compositions. Limonite [FeO.(OH).nH2O] and allophane (a hydrous aluminum silicate) are good examples. Others like volcanic glass and opal (SiO2.nH2O) have short-range order or domains wherein some crystalline-like order exists. Unlike crystalline minerals that show sharp, well defined x-ray diffraction peaks, these mineraloids with short-range order show broad diffraction peaks that give evidence of the short-range order.

Mineraloids meet most, but not all, of the criteria required to be classified as minerals. For example, opals (SiO₂ . nH₂O) are composed of layers of closely packed silica spheres, with H₂O molecules and additional silica filling the void spaces between the silica spheres. Common or amorphous, opal is a mineraloid because the silica spheres have highly variable sizes and thus lack a predictable three-dimensional structure. Precious opal, however, is composed of silica spheres of approximately the same size, producing a predictable three-dimensional array. Precious opals formed through the low temperature inorganic precipitation of colloidal silica from silica-rich waters are classed as minerals; precious opals formed through the accumulation of siliceous tests of silica-secreting organisms are not.

The members of this unofficial class are often mistaken for minerals, but lack the necessary crystalline structure to be classified as such. They are found naturally, are often treated as gemstones, and are included in most mineral references.