How stones are born

Three paths, three families of treasure

Every stone in your collection was born along one of geology's three great paths. Understanding which one is understanding why an amethyst is found inside a volcanic geode, why rubies are born in marble, and why opal is deposited drop by drop.

The igneous path: fire

Deep in the Earth, magma rises, laden with silica, water and rare elements. As it cools slowly in pockets and fissures, atoms have time to arrange themselves: crystals grow, sometimes over millennia. In the cavities of cooled lava, geodes line themselves with amethyst crystals; in pegmatites — pockets where residual magma concentrates water and rare elements — giant crystals several metres long are born.

Typical stones: amethyst, citrine, topaz, tourmaline, aquamarine, peridot. And obsidian, born on the contrary from a flash cooling so brutal that no crystal had time to form: it is a glass, not a crystal.

The metamorphic path: pressure

An existing rock is buried by the movement of tectonic plates. Crushed under kilometres of rock and heated to several hundred degrees, it transforms without melting: its minerals recrystallise into new species. Where limestone meets heat and traces of chromium, rubies are born; in compressed schists, garnets.

Typical stones: ruby and sapphire (in marble), garnet, jade, lapis lazuli, andalusite — the children of pressure.

The sedimentary path: water and time

At the surface, water goes to work: it erodes, dissolves, transports minerals. In fissures and water tables it lays down its mineral load drop by drop, layer by layer, over millions of years. Evaporation concentrates the solutions: silicas become opals, coppers become malachites with their characteristic concentric rings.

Typical stones: opal, malachite, azurite, turquoise, selenite, calcite. And amber, a case apart: it is not a mineral but fossilised tree resin, a plant memory 40 million years old.