Why are Rubies heated?

Why are Rubies heated or heat treated as it is also known? Lets explore some of the reasons
Some terms you will hear, old heat, new heat, lead glass filling, clarity enhanced……so many trade terms.

First lets look at unheated Rubies. What do they have inside them that makes us want to modify and treat them?
-The main visual culprit is silk (or rutile needles). These are tiny needles inside the Ruby that can interfere with light return inside the Ruby and make it look “sleepy”.

Why are rubies heated

-Then there are crystal inclusions in the Ruby, which like silk interferes with the light path. But crystals are obviously more visible and hence the stone is more included.
-The other culprit is nature. Sometimes, we might think that nature has not completed it’s job properly. Ruby, like all corundum, is made of Aluminium Oxide. The red colour in Ruby comes from Chromium. Now when the Ruby forms, there might be defects in the crystal lattice of the Aluminium Oxide which has prevented the correct bonding of the lattice with the Chromium. The Chromium might still be in the Ruby lattice, just not arranged correctly.

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So here comes humanity always looking for perfection. lets chuck it in an oven and see what happens. Results
– Rutile silk becomes dotted and actually melts inside the Ruby. This is good, making the Ruby look more vibrant and reducing the interference with the light path

Why are rubies heated

-Crystals can actually melt inside the Ruby, changing them from solid objects into liquid objects which are more transparent. Crystals can also become white, but I will talk about that latter when I talk about Sapphires.
– Nature….well we give it another chance. If the Ruby is heated so much that the Aluminium Oxide almost melts, it gives everything another chance to re-crystalize and absorb the Chromium in the Ruby better. This improves the colour of the Ruby.
Here are some pictures of my Rubies that I took for you. These pictures have been taken after I had my Rubies heated. The first one is a picture of a “fingerprint” (partially healed fissure. In a unheated Ruby these are filled with a nice liquid and are transparent). This fingerprint is from a heated Ruby. You can tell it is heated because the fingerprint looks whitish (think about the liquid inside the fingerprint evaporating when it got heated) and it now looks like “rain drops on a windshield” instead of like a human fingerprint.

Why are rubies heated

So that’s why we heat them. But…there is more.
What I have spoken about above is “old heat”. That is get the Ruby and heat it by itself. However, “old heat” has a side effect. Imagine putting 2 or more Rubies in a furnace and heating it to almost the melting temp of the Ruby. What happens? They stick together of course. This causes all sorts of problems, including making them brittle. So the cookers coat them in Borax which insulates the Rubies when the get heated. However, the Borax creates another problem. It too can melt. When it melts, it enters fissures in the Rubies and fills the cavities. THIS IS NOT GLASS FILLING. No lab will call this glass filling. This is a side effect of using the borax compound. In the pictures below, I have a heated Ruby. It has a few small fissures on the surface. It is clear to see the borax glass filling in this Ruby. It looks just like….well glass. This is much more stable than lead glass filling and is accepted in the industry as being referred to as “old heat” or heat only.

Why are rubies heated

Why are rubies heated

New heat is another term used. It refers to adding in some other elements when you heat the Ruby. It can be Berylium, or in the case of Rubies, Chromuium can be added to “surface diffuse” the Ruby and making it a very rich colour. This is extremly difficult and is generally done with Sapphires, not Rubies.

Then of course lead glass filling. Take an opaque horrible looking Ruby and make it beautiful. These are acid treated, heated and then placed into the lead glass treatment process. Here is a picture of a lead glass filled Ruby I have for you. See the giant gas bubble, and also the blue and orange flash.

Why are rubies heated

So that’s Rubies. Now a quick note on Sapphires. Lets just look at blue Sapphires.
Same principle, Aluminium oxide plus Titanium and Iron makes blue Sapphire (Ruby was Chromium, Sapphire is Titanium and Iron).
Remember those Rutile needles I spoke about. What are they made of???? Titanium. Woohoo
Since Sapphire needs Titanium to be blue, if we heat the Sapphires, we can actually melt the Rutile needles back into the lattice and make the sapphire more vibrant while also increasing the clarity. Win win!
Again that is called old heat.
New heat for sapphires is adding Beryllium or adding Titanium. Now Titanium is a very heavy element and a very large atom, so making it penetrate to the center of the Sapphire is hard, thats why “surface diffused” sapphires have a thin layer of colour.

Another one of my Sapphires. This one is heated. I took this picture to show you what a heated crystal looks like. See it is white now (befort heat it was reflective) and it has a “halo” around it. Perfect example of what happens to a crystal when it is heated. It literally explodes, creates a fracture plane and injects melted crystal into the fracture plane making the “halo” effect. Pretty awesome.


So there you have it, the basic answers to the question why are rubies heated!

2 thoughts on “Why are Rubies heated?

  1. Brent Nelson says:

    Great article!

    Does the presence of inclusions or impurities jeopardize the final product appearance in any way? Or the integrity of the stone?

    • SedaGems says:

      In most cases, the appearance of a stone will improve after heating because the inclusions inside literally melt.

      Indeed when some stones are heated, they explode because of the stresses caused inside the gem by melting and expanding crystals. once the process is finished, as long as there are no surface reaching fissures, the stone will be stable.

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