Carbons, glasses & organics

What holds the family together

A mineral, strictly, is a naturally occurring inorganic solid with a definite chemical composition and an ordered crystal structure. Most of this family fails at least one of those tests.

Obsidian and moldavite are glasses, cooled too fast for atoms to arrange themselves. Opal is packed silica spheres with water between them, ordered enough to diffract light but not crystalline. Amber is fossilised tree resin, an organic polymer. Jet is fossilised wood. Pearl is aragonite platelets laid down by a mollusc with an organic binder.

Diamond and shungite are both carbon and both genuinely mineral, but they sit at opposite extremes: diamond is carbon in a rigid tetrahedral lattice at hardness 10, and shungite is a carbon rock containing fullerenes, soft, black and electrically conductive in its highest grade.

The range of hardness across this page runs from pearl at 2.5 to diamond at 10, the widest of any family here, which is why the care advice cannot be generalised.

How it forms

Diamond forms at depths of one hundred and fifty kilometres or more and is carried up in kimberlite pipes by violent volcanic eruptions. Most diamonds are between one and three billion years old.

Obsidian forms wherever high-silica lava cools quickly, and specific flows were traded across hundreds of kilometres in the ancient Mediterranean, Anatolia and Mesoamerica; geochemical fingerprinting now lets archaeologists trace individual artefacts to their source flow.

Moldavite formed roughly fifteen million years ago when a meteorite struck what is now Bavaria, throwing molten material hundreds of kilometres into Bohemia, where it fell as green glass.

Opal forms from silica-rich groundwater in arid regions, principally in Australia, and in Ethiopia.

Amber comes from Baltic, Dominican and Burmese deposits, aged from about twenty to over ninety million years.

Shungite comes from one region of Karelia in Russia and is around two billion years old.

The stones of this family

• Diamond (C, Mohs 10). Pure carbon, the hardest natural substance, and the stone whose modern meaning was written by an advertising campaign in 1947. Its supply chain deserves your attention.
• Shungite (carbon, up to 98%, Mohs 3.5). A carbon rock from Karelia containing fullerenes, with real documented use in water filtration and a large volume of unsupported claims about radiation.
• Obsidian (volcanic glass, Mohs 5.5). Not a mineral but a glass, cooled too fast to crystallise. Aztec priests scried in polished mirrors of it, and its edge is sharper than a surgical scalpel.
• Moldavite (tektite glass, Mohs 5.5). Glass thrown out by a meteorite impact in Bavaria fifteen million years ago, found only in Bohemia. Scarce, expensive, and faked at industrial scale.
• Opal (hydrated silica, Mohs 5.5). Not a crystal but packed silica spheres that split light into colour. Held unlucky in the nineteenth century because of a Walter Scott novel, which is the tradition's best cautionary tale.
• Amber (fossil tree resin, Mohs 2.5). Fossil resin, thirty to ninety million years old, that holds a static charge when rubbed. The Greek word for it, elektron, is where electricity got its name.
• Jet (fossilised wood, Mohs 3). Fossil wood, light and warm to the touch, worked at Whitby since the Bronze Age and made into an industry by Victorian mourning dress.
• Pearl (aragonite & conchiolin, Mohs 2.5). The only gem made by a living animal, and the only one that needs no cutting. June's birthstone, ruined by perfume, vinegar and time.

Trade names and lookalikes

Every stone here has a substitution problem.

Copal is young resin sold as amber, and it softens under a drop of acetone while amber does not. Plastic and glass amber is common; real amber floats in strong brine and takes a static charge.

Synthetic opal has been produced since the 1970s and shows a distinctive columnar or lizard-skin structure under magnification. Doublets and triplets are thin opal slices backed and capped, legitimate when disclosed.

Moldavite is faked at industrial scale with green bottle glass, and genuine material has a distinctive etched surface texture that fakes reproduce poorly. Prices for real moldavite have risen sharply, which has made the fakes worse.

Jet is imitated by vulcanite, plastic and glass. Shungite is imitated by ordinary black shale, and elite grade genuinely conducts electricity.

Diamond has cubic zirconia and moissanite as simulants, and laboratory-grown diamond, which is real diamond and should be sold as such.

Care that follows from the structure

No single rule covers this family, so read the stone rather than the page.

Diamond takes anything short of a hammer blow on its cleavage plane and can be steam cleaned.

Obsidian is glass at 5.5 and chips at its edges, which are genuinely sharp; handle knapped pieces carefully.

Opal contains water, can craze if it dries out or suffers thermal shock, and doublets and triplets must never be soaked.

Amber at 2.5 scratches easily, dissolves in solvents including perfume and alcohol, and darkens over decades.

Jet is light and soft and chips.

Pearl is the most delicate object here: perfume, hairspray, vinegar and sweat all etch it. Last on, first off, and wipe after wearing.