Ok, Ok, I keep reading that there is no such thing as
'black onyx'. If that is true then what do I have that is as
black as it gets and is chalcedony? It is not dyed.
Most of the commercially sold cut stones that are sold as black
onyx in the jewelry industry are a black dyed chalcedony. Most
of it started out as a rather nondescript greyish color. They
are often dyed AFTER cutting, and sometimes the dye doesn't reach
all the way through.
However, the statement made in this thread that there is no such
thing as natural black onyx is not quite true. While fairly
rare, and sometimes a bit costly, naturally black appearing
chalcedony is indeed sometimes found. In most of the cases I've
seen, it's not actually completely black, but is instead a very
dark brown, or even greenish. However, it appears black since
it's so dark, plus the fact that it's opaque. Placed next to the
common dyed commercial material, sometimes the natural doesn't
appear as intensely black...
Also, some carnelian type material, which is commonly heat
treated to intensify the brown/red colors, turns such a dark,
opaque brown as to appear for all practical purposes, black.
While not actually dyed, this material can't quite be called
natural black, either, since it's been heat treated...
Telling the difference? Hah. Usually means you have to know
the miner... The dyed material is black due to plain old carbon
left in by the dying process. Not so easy to test for by ordinary
gemological tests... If you don't already know, for sure, that a
piece is undyed, it's usually safest to assume it's dyed. Most
often, the assumption will be correct. For those who insist
it's always dyed, I'd offer, as illustration, the many examples
of montana agate that appear clear to whiteish, only with intense
black markings and patterns inside. Those inclusions are, though
small areas in most stones, natural black chalcedony, right? And
I've got a few small cut stones of such material where the whole
stone is from a larger bit of such an inclusion. Occasionally,
the black areas of that material can be large enough to do that.
I offer that only as an example. Most of the usable black
chalcedony that I've seen is not of that type...
One general rule of thumb I've found useful in dealing with
gems... The words "never" and "always and "impossible" are
usually unwise choices in a description... Often they tend to be
at least slight overstatements...
Hope this helps.