The mineral chiastolite's a variety of andalusite with the chemical composition Al2SiO5. It's noted for distinctive cross-shaped black inclusions of graphite. In areas around Georgetown, California, metamorphosed sediments contained andalusite and chiastolite in a graphite rich metasediment. The chiastolite crystals have been pseudomorphically altered by a mixture of muscovite, paragonite and margarite. The calcium rich margarite tends to form along the graphite rich crosses or bands within the… (More on Chiastolite)