Chiastolite

Chiastolite is a variety of the mineral andalusite with the chemical composition Al2SiO5. Noted for distinctive cross-shaped black inclusions of graphite. The presence of the cross caused this mineral used as a gem. Chiastolite specimens distributed throughout Europe from the 16th century, as an amulet or souvenir provided by the pilgrims returning from Santiago de Compostela (Saint James of Compostella), in Spain.

There are several theories on the formation of the chiastolite cross, however the most accepted theory, proposed by Frondel in 1934, suggests that there is a selective attachment of impurities in the rapidly growing corners of andalusite crystals. As the concentration of these impurities (which consist primarily of graphite) increases, the growth of the crystal slowed. This concentrated impurity deposit forms a re-entrant as it absorbed by the growth of the andalusite porphyroblast. The cycle of growth-retardation-growth, then repeats itself, creating a featherlike pattern of graphite along four radiating ‘arms’.