The Topaz from Ouro Preto Brazil (imperial topaz) is slightly
different mineralogically and chemically than topaz from other
worldwide localities. It’s crystal habit, color stability &
coloration differ from the more ordinary topaz.
Most topazes are very light sensitive and fade in direct sunlight.
The Ouro Preto imperial topaz as a general rule does not. Imperial
from Ouro Preto also has a unique pure orange to red to fuchsia
coloration devoid of the brown, gray and bluish-green tones so
commonly found from nearly all other locations. The crystal habit of
imperial topaz shows a longer c-axis in relation to the A & B axis
than other locales and also has striated prism faces while other
locales do not. Somewhere in the past I read in a credible source
that the actual chemical & physical properties of the Ouro Preto
topaz is different than other localities. Something to do with the
hydroxyl group (OH) and the pH during formation if I remember
correctly, and slight differences in RI and SG.
Anyone who is familiar with the Ouro Preto rough or crystals will
immediately recognize this locale by very apparent and unique
properties. It almost looks like a different mineral species when
compared to common topaz. As I type this - I have in front of me a
Thomas Range Utah crystal, a San Luis Potosi, Mexico crystal (often
erroneously referred to as Guerrero), a Tarryall Colorado crystal
and Ouro Preto imperial crystal. The first 3 materials all have
many similarities in their crystal habit that the Ouro Preto
Imperial material does not share. Hard to sum up in words, but very
apparent in person.
Even amongst the material from the Ouro Preto area there are very
specific mines each with its own and unique attributes.
Here are a few:
a… The Rodrigo Silva mine produces natural super colors approaching
fuchsia-magenta.
b… Antonio Pereira produces large very orange with little dichroism
in the crystals.
c… Vermelao ( I forget the exact spelling but the town behind the
old bauxite smelter) produces huge up to brick size salmon red-pink
colored crystals.
d… And there are other mines around Ouro Preto that produce their
own unique varieties.
Pakistan produces the material most similar to imperial topaz from
Ouro Preto. It approximates the color of some of the Brazilian
material but not the best. Overall the Pakistani size and quality
does not come close to what Ouro Preto has produced. I am not sure
if it is color stable in strong sunlight or qualifies to be called
imperial topaz. Of course the Pakistani dealers all call it
imperial.
Anyways what I stated above is based on my visits to Ouro Preto and
dealings with the material and mine owners. My knowledge of the
Pakistani material is from shows and written materials. A lot of my
topaz knowledge is from my years spent in the mineral business
seeing specimens from many locales, talking with mine owners and
Brazilian dealers. The largest and one of the most prolific mines in
Ouro Preto is actually owned by and American mineral collector who
is quite wealthy and keeps all in his private collection. And what a
collection it is. Sometimes he puts it on display in Tucson at the
Main Show. Maybe you’ve seen it? Hope this helps somewhat.
Steve Green / Rough and Ready Gems, Inc. www.briolettes.com