f019253d_The_Delaware_Geological_Survey_inerals_of_the_Red_Clay_Valley_Description
[Rock Type] Amphibole [Description] 
A large family of minerals. In the Delaware Piedmont, they are usually black or dark green. Amphiboles usually have one good cleavage that will sparkle on fresh surfaces. Arock containing around 50% or more amphibole is called an amphibolite.
Common Rocks and Minerals of the Delaware Piedmont | The Delaware Geological Survey
Description