Staurolite Schist

Staurolite schist is a medium-grade metamorphic rock consisting of a foliated mica matrix (muscovite and biotite) embedded with prominent porphyroblasts of staurolite. The staurolite crystals are dark red-brown to nearly black and often stand out as stubby prisms against the silvery, glittering schist.

The rock is best known for staurolite's famous cruciform (cross-shaped) twins, popularly called "fairy stones" or "fairy crosses," which have been collected as natural good-luck charms for centuries, notably in Virginia, USA.

Staurolite schist is a classic index rock for geologists, marking a specific window of temperature and pressure in regional metamorphism.

Staurolite schist forms by regional metamorphism of aluminum-rich (pelitic) sedimentary rocks such as shale and mudstone. As burial heating reaches medium grade (roughly 500-650 C), the parent clays recrystallize into mica while iron and aluminum combine to grow staurolite porphyroblasts.

Staurolite is an index mineral defining the staurolite zone of Barrovian metamorphism; it commonly coexists with garnet, kyanite, and biotite. Directed pressure aligns the platy micas into the schist's foliation while the tougher staurolite grows across it.

Classic occurrences include the Appalachians (Virginia, Georgia, New Mexico), the Scottish Highlands, the Alps, and Brittany, France.

Look for a foliated, mica-rich schist that splits along glittery planes, dotted with hard, stubby, reddish-brown to brownish-black staurolite prisms and sometimes red garnet. The staurolite crystals are notably hard (~7) and may show the diagnostic cross-shaped twins.

The schist matrix is softer and silvery; staurolite has a resinous to vitreous luster, a white to grey streak, and resists scratching. Distinguish staurolite from garnet (which is rounded, more equant, and harder, ~7.5) and from dark tourmaline (which forms slender striated prisms). The cruciform twin is unique to staurolite.

Staurolite schist itself has little industrial value, but its staurolite crystals, especially the cross-shaped fairy stones, are popular collector and curio items sold as natural amulets and jewelry.

Geologically, the rock is important as a metamorphic index rock used to map the temperature-pressure conditions a terrain experienced. Staurolite is also the state mineral of Georgia, USA, reflecting its cultural following.

In folklore the fairy cross is a protective talisman, though this is cultural rather than scientific. Massive staurolite has minor use as an abrasive sand.