In 2011, I purchased this zircon rough in Tanzania. Zircon from Singeda, Tanzania is often a super-saturated, near-black shade of red. In reflected light, the stone appears near-black, but it is red in transmitted light. I cut the stone in a cut-corner square brilliant of my own design. But this kind of zircon is so dark, it really requires heat treatment to make an attractive gem. I decided to heat the stone incrementally so I could document the color change in progress.
For each heating run, I put the zircon near the center of a crucible filled with alumina. Using a programmable kiln, I then heated the crucible to my target temperature, held it there for 1 hour and then allowed the kiln to cool at it’s own pace, waiting to remove the stone until I could touch the crucible with my bare hands. After each heating run, I photographed the zircon under daylight equivalent fluorescent lighting with a photographic white card as background. The ruler provides scale and aided in positioning.
Before Heating:
after 350C
after 375C
after 400C
after 425C
after 450C
after 475C
after 500C
