Thought this may be of interest. This is a picture of the rough Tanzanian Zircon... kind of a honey brown color. THe next picture of the cut stone after low temperature heating. The heating is actually done just over a candle flame for a few minutes. You can literally watch the color change.
Zircons are funny stones, and fun stones to play with. If you dont heat them enough, after they cool they will come back to the original color. If you heat them too hot, then they have a good chance of going clear. Once you get them to a color, and it stays a few hours after heating it will be permanent, unless someone heats them again.
I have seen blue zircons turn brown from just a suntan booth. This stone in the picture has been this color now for over a week, so it will stay this way.
What's different about heating a zircon compared to maybe a tanzanite is the temperatures are much lower, and the change in color happens very quickly. For this reason it's always best to heat them after cutting, and by hand over a flame so you can watch the change and take it off the heat. Not all zircons will change, and not all that do change will change to a nice color.
Is it possible to heat it unevenly somehow to get a bi-colored effect? Thanks for sharing that info with us---pretty neat. I had no idea it could be done with zircon. I've accidentally changes an amethyst to a greenish color before by leaving a stone on the heating pad for too long, but it wasn't really all that plesant of a color, unlike your zircon.
It's amazing to read these threads and find out all sorts of interesting things about gemstones!!
Imagine! add a little heat and presto! you get a different color! I'll be sure to keep my zirons away from heat!
Thanks for all this info guys!