Andalusite

Andalusite, a mineral consisting of silicate of alumina, occurs in slightly rhombic, four-sided prisms, and also in a massive form; pearl-gray or reddish in colour. It is met with not infrequently as a constituent of certain metamorphic rocks, as Andalusite-slate. A variety of andalusite is known as chiastolite (Gr. chiastos, 'decussated'). The crystals of this variety, when broken across, often show cruciform or tessellated patterns. They are common in certain altered slates.