Chiastolite

Chiastolite is an opaque, grey or whitish variety of the mineral andalusite. As the crystal grows, graphite (carbon) and clay are trapped along its diagonals, so a cut across the crystal reveals a dark cross or “X”. The name comes from the Greek chiastos, “arranged diagonally”, and refers to the Greek letter chi (χ), which looks like an X.

Amulet and religious symbol: Its natural cross made it an amulet and pilgrim’s keepsake for centuries, and it is still used in jewellery as a Christian symbol. Ornamental stone: Slices cut across the crystal and polished are made into pendants, earrings and cabochons; the town of Laraquete in Chile is known for handmade jewellery of chiastolite, called “piedra cruz”. Collecting: Large crystals with a clear pattern are sought by mineral collectors.

It has a hardness of 6.5–7.5 on the Mohs scale. It scratches glass and steel; a steel knife cannot scratch it. Its specific gravity is 3.13–3.17 g/cm³.

Its best-known localities: Boal, Asturias, Spain; Mount Howden (north of Bimbowrie), Olary, South Australia; Santiago de Compostela, Galicia, Spain; Lancaster, Worcester County, Massachusetts, USA; Carreço, Viana do Castelo, Portugal.

It forms prism-shaped grey or whitish crystals with an almost square cross-section; a cut across the crystal shows a black cross, an “X” or a checkerboard-like pattern inside. The pattern changes along the crystal: according to Kunz, slices from one end first show a white cross outlined in black, then the black cross grows stronger further along; no two crosses are ever exactly alike. It occurs as rods embedded in slate, schist and hornfels that were heated around a hot body of magma such as granite. Staurolite twins are also sold as “cross stones”; they are not chiastolite but a different mineral.

On the “street to diamond” value ladder it sits on step 5 of nine: Ornamental stone. Chiastolite is an inexpensive ornamental and collector’s stone; its value depends above all on how clear and symmetrical the cross is: a sharp black cross with even arms on a pale background is the most sought after. A good polish, freedom from cracks and size also matter for cut slices. Being opaque, it is not faceted but worked into slices, cabochons and pendants. Large crystals from famous localities such as Mount Howden or Lancaster are sought by collectors.

According to Kunz, chiastolite worn touching the skin was once believed to stop bleeding from any part of the body and to increase the flow of milk, and to cure all kinds of fever when hung from the neck. The same source says that, because its cross was taken as a holy sign, chiastolite was believed to drive evil spirits away from the wearer. From the 16th century, pilgrims returning from Santiago de Compostela carried chiastolites across Europe as amulets and souvenirs. There is no evidence that chiastolite stops bleeding, increases milk, lowers fever or drives away evil spirits; bleeding and high fever need medical help. The cross is an entirely natural pattern, formed as graphite and clay line up along the diagonals while the crystal grows. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.