I BELIEVE that they're ALL treated. Is that incorrect???
That is a safe assumption, yes. Most of the material that comes out
of the ground a black color (called Carbonado) is an aggregate, and
isn't cuttable (as an aggregate, there are no cleavage, sawing or
ploshing planes). Typically, diamonds that are brown, yellow and
somewhat black, and are too heavily included to cut a decent-looking
or durable stone, are irradiated to make them as black as possible.
They are then tested for radioactive levels. Any irradiated stones
with unsafe levels are stored until their half-life expires to safe
levels. If I ever shop for them at shows, I know a guy with a geiger
counter (and I'm not kidding - I'd borrow it to be sure!).
I just read that the West Australia state government has agreed to
help Rio Tinto with the half-Billion-dollar price tag of
redeveloping the Argyle mine for underground mining, pending
inspection by an independent auditor to determine whether Rio Tinto
actually needs the money. The Argyle mine, while producing the few
ultra-expensive pink/purple/red diamonds, is as well known for their
usually small yellow (champagne) and brown (cognac) stones, as well
as the small black ones we've been talking about. Perhaps prices
will drop in a few years if and when they go underground.
James in SoFl