Staurolite is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Orthorhombicly structured gems are made of aluminum iron hydroxysilicate, their full chemical compound being (Fe,Mg,Zn)2Al9(Si,Al)4O22(OH)2.
Staurolite is a hydrous magnesium aluminum silicate that crystallizes in the monoclinic system.
It appears as coarse, dark gold-brown prisms, or sometimes reddish-brown to black stubby crystals. On weathered rock, it stands out in contrast like sand-coated yellowish brown prisms. Surfaces are often rough or covered with an earthy coating because of natural alteration.
It frequently occurs in characteristic cross-shaped twins, interpenetrated so that its intersection is finely beveled. The cruciforms are in the three-dimensional figure of of either a Greek cross (90° angle between the arms) or of a St. Andrew's cross (60° angle). It may appear as shapeless dark-brown granules in rocks, but very rarely in massive forms. It oftens also occurs growing parallel with kyanite.
It is very hard, semi-opaque, rarely transparent or translucent, with vitreous to resinous luster. It is infusible and virtually insoluble, though sulfuric acid will affect it slightly. Some varieties have manganese traces, in which case they will fuse.
Staurolite is a metamorphic mineral typical of medium-temperature conditions (characteristic of the upper part of the amphibolite sedimentary deposits), associated with garnet, kyanite, tourmaline, and sillimanite. Occasionally it is found in some pegmatites and contact metamorphic rocks. Because of its hardness and insolubility it is also often found in alluvial sands.
Reddish-brown translucent crystal prisms associated with blue kyanite and gray paragonite occur in mica schists at Pizzo Forno (Ticino, Switzerland). Large crystals are also found in Goldenstein (Moravia), Bavaria (Germany), Scotland, and various places in the USA. Cruciform twins are found in Fannin County, Georgia, at Pilar, New Mexico (USA) and Morbihan (France). In Italy, large crystals are common in schists at Monte Legnone (Como) and in Valtellina.
A prominent specimen of staurolite is the Basel Baptismal font, a cruciform twin of staurolite, used as an amulet in Basel, Switzerland.
Lusakite is a variety of staurolite consisting of cobalt.
Other names for staurolite include: cross stone, crucite, grenatite, lapis crucifer, lucky stone, staurotide, and twin stone.
The term staurolite comes from the Greek word "stauros" meaning "cross" as this mineral is famous for its cross-, plus-, or X-shaped interpenetrating twinned crystals. Because of its rarity and its unique cross-shape, it has been a seed for many stories in several cultures about where staurolite has come from, what it is, and what it can do. Some say the shapes are actually fragments of a meteorite that fell to Earth, or artifacts from the time of the crucifixion of the Christ. Others say its form and composition show that it comes from deep within the Earth, transported to the crust by thousands of years of tectonic shifting and volcanic activity. But regardless of the different theories about its process, staurolite pieces are used across several cultures as faith crosses, believing that it would bring them safety and well-being.
Plus-shaped twins and dark brown, transparent crystals are sometimes used as gemstones. Cruciform twins are worn as religious jewelry. Clear crystals are used to a small extent as cut stones.
Staurolite crystals are almost never transparent, and are frequently found in very dark colors. Pieces for gem-cutting are always small, less than 2 cts in general, which are found in Brazil and Switzerland.
Cross- or plus-shaped twins are known as "fairy stones" in North Carolina (USA). These items are sold as amulets, although taking advantage of the demand, many of those labeled as staurolite on the market are dyed imitations carved from similarly-textured rock.
This mineral is important in petrology as a means of defining the metamorphic type and grade of the host rock, because staurolite is estimated to be several hundred million years old.