Chiastolite

Chiastolite is a variety of the mineral andalusite (Al2SiO5) distinguished by a characteristic cross-shaped pattern of graphite inclusions within the crystal. The cruciform chiastolite cross results from carbonaceous material arranged in four directions during crystal growth, producing a dark cross that stands out against the lighter background of the mass.

In hand specimens, chiastolite crystals typically display a pale gray to brown host with a prominent black cross when viewed in a plane that reveals the inclusion geometry. The cross is most distinct in polished faces or fresh surfaces. Andalusite, including chiastolite, has a hardness around 7 on the Mohs scale and occurs in a range of metamorphic rocks, particularly aluminous pelites and schists, formed under low to medium-grade conditions. The presence of graphite or other carbonaceous inclusions is essential for the chiastolite pattern and is not a universal feature of all andalusite.

Etymology derives from chiastos, the Greek word for cross, reflecting the defining cruciform pattern. Chiastolite crystals are sought after by collectors and can be cut for ornament or studied as a diagnostic variety in metamorphic geology. Although not as widely used as some gemstones, high-quality chiastolite specimens with a clear, well-defined cross are valued for their distinctive appearance and for illustrating mineral-inclusion patterns produced during metamorphism. Notable occurrences are reported in various metamorphic belts around the world, where andalusite-bearing rocks provide the conditions for chiastolite formation.