Mineraloid.

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 International Mineralogical Association reserves the word mineral for a naturally occurring inorganic solid with a definite chemical composition and an ordered crystalline structure. A mineraloid usually fails one or both of the last two tests. Some are amorphous: their atoms are not arranged in a repeating lattice. Some are organic in origin, formed by plants or animals rather than by inorganic geological processes. Either way, they sit just outside the definition while still being beautiful, durable enough to wear, and important to crystal practice.

The familiar mineraloids

The most common examples are opal, amber, jet, pearl, obsidian, mercury, and tektite. Opal is a hardened gel of silica spheres (hydrated silicon dioxide, SiO2 with five to ten percent water), which is why it shimmers with play-of-colour but lacks the geometric form of quartz. The diffraction grating formed by the regular packing of those spheres is what produces the rolling colour, and it disappears the moment the spheres are uneven. Amber is fossilised tree resin, often from the extinct conifer Pinus succinifera, with the best material from the Baltic dating to around 44 million years ago. Jet is a compact form of fossilised wood, related to coal, polished into mourning jewellery during the Victorian period after the death of Prince Albert in 1861. Pearl is built from layers of aragonite (a calcium carbonate that is technically a mineral) and conchiolin protein, secreted by molluscs in a structure called nacre; it is organised, but not as a single crystal lattice. Obsidian is volcanic glass, cooled too quickly for crystals to form at all, with a hardness of about 5 to 5.5 and a famously sharp conchoidal fracture; obsidian blades are still sharper than surgical steel and have been used in some experimental scalpels. Tektite and moldavite are natural glasses formed by meteorite impacts, melting terrestrial rock so quickly that no crystal lattice could grow.

A few odd cases sit at the edge. Mercury is a liquid at room temperature, so it is technically a mineraloid by the structure rule, even though it is otherwise treated as a mineral. Coral and ivory, like pearl, are organic. Shungite, a strange carbonaceous rock from Karelia, has been argued both ways depending on how strictly the lattice rule is applied. The category is a useful drawer rather than a sharp wall.

Care and use

For crystal work the distinction is mostly a matter of vocabulary and care. Mineraloids tend to be softer than true crystalline gemstones (opal sits at 5.5 to 6.5 on the Mohs scale, amber at only 2 to 2.5, jet at 2.5 to 4, pearl at 2.5 to 4.5) and often more sensitive to heat, light, and water. Opal can dry and craze, especially Ethiopian hydrophane opal which actually drinks water and changes appearance when wet. Amber scratches easily, melts at a low temperature, and dissolves in some solvents. Pearls are damaged by perfume, hairspray, and sweat (the rule of thumb: pearls go on last and come off first). Obsidian chips with a sharp edge that can cut skin if handled roughly. Jet picks up scratches from anything harder than a fingernail.

A common misconception is that mineraloids are "fake" crystals, somehow less authentic than quartz or amethyst. They are not. They are simply formed by different processes, and the rules of mineralogy were written before opal and amber existed in the human record. Some of the oldest gem material on the planet sits in this category: amber far older than any cut diamond, pearl that was traded across the ancient Mediterranean. 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.