Larvikite

Larvikite is not a single mineral but a rock: an igneous rock, a variety of monzonite, named after the town of Larvik in southern Norway (some sources call it a syenite). It formed deep underground as magma cooled slowly, and it is made mostly of large feldspar crystals, roughly thumbnail-sized, with a smaller share of dark minerals.

What catches the eye is the silvery-blue flash. Inside the feldspar crystals are very thin, alternating layers of two kinds of feldspar. Light reflected from these layers interferes and produces the shimmer, known as the schiller effect or labradorescence. Tilt the stone in the light and it changes its mood.

The rock formed in the Permian period, about 292 to 298 million years ago, in the Oslo Rift, a region where the continent began to split apart. According to the International Union of Geological Sciences (IUGS), it is one of the few places in the world where magmatism deep beneath a continental rift can be studied. Outside Norway, larvikite is also found near Thunder Bay, Ontario, in Canada.

Most of the world's larvikite never reaches a crystal shop. It ends up on building facades. It is polished into tiles and kitchen countertops and sold under the trade name "Blue Pearl Granite", even though geologically it is not granite at all. Amsterdam's Schiphol Airport, for example, has walls clad in it. Next time you wait in the lobby of a fancy office building, take a look at the walls.

Larvikite is Norway's national rock. The IUGS recognized it as a Global Heritage Stone Resource, and it was one of the first rocks to receive that status.