A multicolored mat of needle-like sillimanite crystals surrounds a single brown crystal of biotite. You can also see some long, slender sillimanite crystals enclosed within the biotite. The sillimanite formed when ordinary mud, made of quartz and aluminum-rich clay minerals, was buried, compressed, and heated to high temperature deep in the roots of an ancient mountain range. This pressure cooking changed the mud to the metamorphic rock called schist. Uplift and long erosion have returned this rock to Earth's surface from a depth of roughly 15 miles.

Crossed polarizers