Staurolite

Staurolite is a silicate mineral containing iron, magnesium and aluminium. Its crystals are brown and prismatic; its most striking feature is that two crystals can grow through each other to form twins shaped like a cross or an “X”. Its name comes from the Greek words for “cross” (stauros) and “stone” (lithos). It occurs in medium- to high-grade metamorphic rocks (schist, gneiss) together with garnet, mica and kyanite.

Scientific indicator: Finding staurolite in a rock helps estimate the temperature, depth and pressure at which the rock was transformed, so staurolite is used as an index mineral. Charm and keepsake: Cross-shaped twins have been carried for centuries as charms, keepsakes and jewellery. Collecting: Well-formed cross-shaped crystals and twin groups are sought after for mineral collections.

It has a hardness of 7–7.5 on the Mohs scale. It scratches glass and steel; a steel knife cannot scratch it. Its specific gravity is 3.74–3.83 g/cm³.

Its best-known localities: Fairy Stone State Park, Patrick County, Virginia, USA; Brittany, France; Fannin County, Georgia, USA; Taos, New Mexico, USA; Pizzo Forno, Ticino, Switzerland.

Known localities in Türkiye: Çine Massif, Aydın, Türkiye.

Brown, short, stout prismatic crystals; the surface is often dull or resinous. Cross-shaped twins are typical: “X”-shaped twins meeting at about 60° are more common, twins meeting at 90° in a Roman or Maltese cross are rarer. Most crystals, though, are single, untwinned crystals. Its hardness is 7–7.5: a steel knife cannot scratch it, and it is close to quartz in hardness. Its specific gravity of about 3.7–3.8 makes it clearly heavier than quartz.

On the “street to diamond” value ladder it sits on step 4 of nine: Fine dimension stone and ore. Staurolite is very common, so its value is low. The most sought-after pieces, among collectors and the public alike, are well-formed cross-shaped twins: the symmetry of the arms, undamaged crystal faces and a well-known locality add to the value, while untwinned single crystals attract much less interest.

According to George F. Kunz, the Swedish mineralogist Cronstedt wrote in 1758 that in Basel staurolite was used as an amulet at baptisms and was therefore called “Baseler Taufstein” (baptismal stone). According to Kunz, in Brittany these twin crystals were worn as charms and, by local legend, were said to have fallen from the sky. According to a legend told in Patrick County, Virginia, fairies who heard the news of the crucifixion of Christ wept, and their tears crystallised into little stone crosses as they fell to the ground. “Fairy crosses” are regarded as good-luck stones; Presidents Theodore Roosevelt and Woodrow Wilson are rumoured to have carried them. The cross shape comes from a twinning the crystal makes as it grows; the stone did not fall from the sky, and fairies did not weep. There is no evidence that staurolite protects anyone or brings luck, but the “fairy cross” carries real value as a folk tradition and keepsake. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.