Mineraloid

A mineraloid is a substance that resembles a mineral but lacks a crystalline structure. Unlike minerals, mineraloids do not show long-range order in their internal arrangement, and their properties can be more variable. They may be inorganic or derived from organic materials and are typically formed by rapid cooling of molten material, deposition of amorphous gels, or other processes that prevent crystal lattices from developing.

Common examples include opal, a hydrated amorphous silica; obsidian, natural volcanic glass; fulgurites, glassy bodies formed by lightning strikes; and tektites such as moldavite, which are natural glasses produced by meteorite impacts. These materials are often glassy or microcrystalline in texture and can fracture conchoidally.

Because they lack crystalline order, mineraloids do not fit the strict mineral definition, and their exact composition can vary. They are distinguished from true minerals by their lack of a periodic atomic arrangement, though they can be chemically complex and heterogeneous.

The term mineraloid emphasizes their mineral-like appearance without crystal lattices. Opal, obsidian, fulgurite, and other natural glasses are commonly described as mineraloids in geological literature, reflecting their amorphous or poorly crystalline character.