f019253d_The_Delaware_Geological_Survey_inerals_of_the_Red_Clay_Valley_Description
[Rock Type] Sillimanite [Description] 
Sillimanite, or fibrolite as it is commonly called, occurs as aggregates of thin fibers, nodules, or veins. Its color is either gray blue or dull white. It is a high-grade metamorphic mineral that occurs in the gneisses and granitic pegmatites. Sillimanite is the Delaware State Mineral.
Common Rocks and Minerals of the Delaware Piedmont | The Delaware Geological Survey
Description