I believe Hematite was marketed as Alaska Black Diamond -
although it is not even close in characteristics to a diamond.
There are darker diamonds but this is not typical.
Hematite is a iron oxide Fe2O3 that takes a nice polish and has
a metallic sheen to it. It is called Hematite from the Greek word
ema (->Hema) which means blood. This from the fact that Hematite
when ground produces a red coloured waste.
Island Gem and Rock