kyanite crystals in quartz - São José de Safira, Minas Gerais, Brazil - hand specimen In the field photo

In the field photo

with the black looking just like weathered obsidian

Click images to select

because the grains are relatively large and not well rounded

Both break with a sharp-edged fracture

kyanite crystals in quartz - São José de Safira, Minas Gerais, Brazil - hand specimen In the field photokyanite Al2SiO5 Kyanite forms when aluminum rich rocks are metamorphosed, and is frequently found in schists that formed at medium to high pressures and lower temperatures in areas of regional metamorphism. At higher temperatures, kyanite converts to sillimanite. Occasionally the metamorphism produces a pegmatite with large crystals that form when a fluid rich magma allows rapid movement of atoms. These large crystals of kyanite and the surrounding