A mineraloid is a naturally occurring substance that resembles a mineral but does not meet all the requirements to be classified as one. Most commonly, a mineraloid lacks a long-range crystalline internal structure (it is amorphous).
By definition, a mineral must:
Mineraloids meet most of these criteria, but they lack crystallinity - their atoms are not arranged in a repeating 3D pattern - so they are considered amorphous rather than crystalline.
Mineraloids often display striking visual properties—opal’s play of color, amber’s inclusions, obsidian’s glassy luster—making them desirable for both jewelry and display. Collectors and curators distinguish them from true minerals for scientific accuracy and proper labeling.
A mineraloid is a natural substance that looks and behaves like a mineral but lacks the internal crystalline structure required for full mineral status. Understanding this distinction helps with correct identification and labeling—e.g., differentiating crystalline quartz from amorphous opal, or glassy obsidian from crystalline feldspar.