Garnetite is a crystalline-textured metamorphic rock that is overwhelmingly composed of garnet. Garnetite is quite hard (H = 7) and easily scratches glass. It is a high-grade metamorphic rock that can form in deep subduction zone environments or by contact metamorphism. The sample shown below is a contact metamorphic garnetite.
Garnetite (reddish-brown) with molybdenite (silver, MoS2) & ferrimolybdite (yellow, Fe2(MoO4)3·7-8H2O) from near Brownes Lake, northeastern Beaverhead County, southwestern Montana, USA. This is a contact metamorphic rock (skarn) produced by intrusion of the Pioneer Batholith (Late Cretaceous, 69-77 million years) into Paleozoic limestones.