Pictured above is a flux healed ruby crystal from Mong Shu Burma, cabochon grade. Notice the 'candy coating' of melted Borax.
When I first started looking into gemology and the gemstone industry I couldn't help but notice the topic of Flux Treated Rubies. It was being treated as a new topic however, it may have been going on for several years by then.
What does this term, flux treated, mean? First off, flux is an additive used to help melt a substance at a lower temp than it would without it. We use flux when we solder plumbing pipes or weld steel. In silver and goldsmithing I use flux to help melt my alloys as well as to solder my pieces. The flux I use is simple Borax, not surprisingly, the flux used to treat rubies is often the same.
Coincidentally I built a home 45 minutes from the border of Burma where the rough Ruby material used in flux heated rubies was entering Thailand. The rough was from an area called Mong Shu. It was bluish/purple and badly cracked. Most of it had been unsellable thus far.
Borax had been found to reduce the thermal shock of heating stones, in other words, if you would sprinkle some of this white powder onto your rough stones before you heated them you would have less broken and cracked stones. This knowledge lead to a miraculous discovery. When you added Borax to your severely fractured Mong Shu rough and heated it to near melting point, your rough was no longer fractured!! Plus the blue/purple color had change to a beautiful ruby red!!
Now this was huge, not only because Mong Shu had lots of this rough but because little fine ruby had been found anywhere for a long time. Ruby with great color and no feathers/cracks was very rare.
How did it have no cracks? The flux melted, like it always does, and ran down into the cracks. Here it caused the ruby material on each side of the crack to melt and stick together, very similar to welding/soldering. Now the cracks were 'healed' turning a material from being fragile to being even more durable than a common ruby as they often have some feathers/weak points.
So then the labs had to come up with terms for this in their reports and in my opinion did a poor job of communicating. Some like to call this ruby that melted and welded together, 'synthetic ruby'. That may be scientifically accurate but is misleading to most people as it sounds like a man made ruby was added when it was not. Any natural ruby that has melted can be defined as a synthetic ruby. Others said there was a 'glass filling in the cracks' which is also misleading in my opinion as most of us would understand that to mean the cracks were still there but were now filled with the same thing we make windows out of. Later somebody said 'flux healed' and that seems much more easy to understand.
For years dealers complained about the process as it was certainly more than simple heat treatment but they could not stop the industry from accepting it. The truth was, they were beautiful, durable and still rare. Those are the three things that make a gemstone a 'gem'.
Now I see that the worlds most sought after Ruby rough, Burmese Mogok, is also heated with Borax. The price only seems to be affected at the very top end nowadays. One dealer friend here in Thailand was trying to buy a 2.5 carat ruby from another for $15,000 US dollars but the owner said he was giong to hold out for $25,000. I asked whether or not it was unheated Mogok, the answer was no, flux healed Mong Shu.