Also, there is no such thing as black onyx, it is white onyx dyed pure black, and then marketed as black onyx.
I believe the process for producing black onyx is soaking in hot
sugar solutions then in strong (sulphuric ?) acid which
carbonizes the sugar. The porous (always found it funny that it
was so porous) agate is black to a considerable depth and can be
cut normally. This black is permanent and won’t be affected by
any solvent. There are, unfortunately in my opinion, a lot of
dyed agates about so I wouldnot be surprised to see a dyed black
variety.
Andy Parker, Agate House Lapidary
Ulverston, Cumbria, England
http://www.netcomuk.co.uk/~andyp