This is a large chamosite chlorite pseudomorph after almandine garnet
A pseudomorph is when one mineral replaces another mineral, but retains the original crystal form. So while this stone looks like a garnet, it has a completely “new” chemical makeup.
These were called “Black Diamonds” by the miners. These are heavy, as they contain iron.
Old stock, Collector’s item
491 grams (1 lb, 1 oz)
Locality: Michigamme Mine, Michigamme, Michigan