There are several newer Topaz coatings on the market,I assume you are
not referring to the one I have heard as “Mystic Topaz”,which is
irridescent Green.Mystic is a trade name for a coating that first
appeared on the Hong Kong market that is basicly what is used to coat
sunglasses.It is pretty durable,but it is just a spray plastic or
silicon based coating. Then you might also be referring to Topaz as it
is just released after irradiation.When you irradiate White Topaz,it
comes back from the lab in brownish,or greenish colors.Then it is
heated in just a toaster oven to convert it to Blue.Now you can get a
small percentage that is an attractive Green just out of the lab.I
observed some of the massive manufacturer’s of Topaz seperated these
before heating and marketed “Green” Topaz.This is color all the way
through,but on exposure to heat of 375F for 1 hour,or prolonged
sunlight,it can go Blue! Now more likely,you are referring to what is
being termed in the maketplace diffused Topaz.This is a process in
which a chemical coating,I would guess chromium based,is painted on
the gems,then forced into the lattice of the surface by intense heat
and possibly pressure.This is similar to the diffused sapphire that
was developed some 10 years ago.This process is developed in the
US,there are two large manufacturers of it that I am aware of.I won’t
drop the labs name,but one of the largest producers of it is right out
there in Dr.Asplers back yard,Bangkok.The competitor has 500,000 ready
carats poised to make the market debue.I would guess up until now they
are both using chinese Topaz,as it does not matter which mine of
origin like in irradiation,so a poor treater,the most inexpensive
will do.But me thinks,because of poor preperation,they will look to
Brazil where I heavily influence or control all the White Topaz 
and
soon all the Imperial Topaz. Mark Liccini
It sure sounds like this is what they are calling “Evergreen”, just
my own preference, but I don’t care for diffusion treatements. They
are far better than plastic sprays, but I like the color of a stone
to go all of the way through and be relatively stable.
Ben
Ben’s Gems
My preference too… Just another stone to watch out for…and keep
out of the pickle pot. I have wondered if it is possible to remove
the “diffusion” coating with tripoli, or another abrasive prepolish
if you stay too long/close to the stone? Anyone had that experience
with these or similarly diffusion treated stones?
Brian P. Marshall
Dear Brian,
I don't know exactly what polishing operations would damage
diffusion-treated stones, but I do know that the color only goes
a very short way -- less than a mm, I believe -- into the stone,
so it's certainly possible to polish the color off. (That was
one of the big issues when diffusion treated sapphire was
introduced 10 years or so ago.) Most of the references I've seen
to the problem had to do with intentionally repolishing the
stone itself, and I don't know if a similar problem would arise
during normal jewelry polishing, but I wouldn't be surprised if
it could. At any rate, I'd certainly ask the question of someone
familiar with diffusion treated stones and their properties!
Perhaps someone here on Orchid knows more?
Suzanne Wade