mineraloids

Mineraloids are naturally occurring solid materials that resemble minerals but lack a definite crystalline structure. In geology and mineralogy, a mineraloid is a non-crystalline or poorly crystalline substance that does not exhibit long-range crystalline order. They may be inorganic or organic in origin and can form by rapid cooling of molten material, precipitation from gels or solutions, diagenetic processes, or the fossilization of organic matter.

Common examples include opal, obsidian, fulgurite, tektite, and volcanic glass, as well as amber and jet. Opal is a hydrated amorphous silica; obsidian and volcanic glass are naturally occurring glasses; fulgurites and tektites are glassy products of extreme processes. Amber and jet are organic mineraloids formed from fossilized resin and compressed plant material, respectively.

Mineraloids typically lack a crystalline lattice, so they do not have the sharp, well-defined crystal faces seen in minerals. Their internal structure is often amorphous or contains microcrystalline phases. They can be transparent to opaque and display a wide range of colors. Their chemical compositions are often variable, and some may be hydrous or contain volatiles.

In practice, the term mineraloid helps distinguish these non-crystalline materials from true minerals, though many can be important for jewelry and geology. They are identified by their texture, fracture (often conchoidal), and lack of long-range order, rather than by a crystallographic-metric system.