Yellow topaz is not Imperial Topaz…without getting too technical, Imperial Topaz
is defined in the Gemworld Guide as a saturated orangy pink to pink color.
As far as natural blue topaz, you will see it for sale at a premium compared to
irradiated and heated blue topaz, but I don’t see any authorities stating a
premium price for it. The biggest problem with it is establishing provenance…
usually one sees stones which are too pale to be treated and assumes it to
be natural, but if you are paying a premium price for this, I would assume you
would have to have an established provenance. It mainly comes from Texas,
but I think is also found in Nigeria. I personally would not pay a premium for it.
It’s not that rare and there just isn’t much demand for it outside of Texas, where
locals buy it cut in the Lone Star Cut. -royjohn