Please take note that many black diamond are treated i.e. irradiated
to get that black appearance. As natural black diamond derived its
blackness from numerous carbon inclusions. Moreover, it is quite
difficult to facet black diamond as the crystal orientation make it
more difficult to cut and polish unlike the gem quality diamond.
By the way, if you do jewellery repair on irradiated black diamond,
please be extra careful, as my research showed, that the heat of the
jeweller torch will reverse the irradiated black diamond to brown or
yellowish-brown diamond, their original colour.