Mineraloid.
A naturally occurring solid that lacks the ordered crystalline structure of a true mineral; opal, amber, and obsidian are examples.
A mineraloid is the term mineralogists use for a natural substance that looks and behaves like a mineral but does not meet the strict definition. The usual sticking point is structure. True minerals have atoms arranged in an ordered, repeating crystalline lattice. Mineraloids do not.
The most common examples are opal, amber, jet, pearl, and obsidian. Opal is a hardened gel of silica spheres, which is why it shimmers with play-of-colour but lacks the geometric form of quartz. Amber is fossilised tree resin. Jet is a compact form of fossilised wood. Pearl is built from layers of aragonite secreted by molluscs, organised but not in a single crystal lattice. Obsidian is volcanic glass, cooled too quickly for crystals to form at all.
For crystal work the distinction is mostly a matter of vocabulary and care. Mineraloids tend to be softer than true crystalline gemstones and often more sensitive to heat, light, and water. Opal can dry and craze, amber scratches easily, obsidian chips with a sharp edge. They are well worth keeping. They simply ask to be handled with the awareness that they are something a little different from a quartz point.