Asterism and heat treatment

Apparently, some stars are heated nevertheless (like this) so no blanket statement applies...

Down that link there''s a short explanation of how the particular kind of heating is done.  I am affraid I do not know more myself.

Hi
Star sapphire in our field is rare but my understanding concurs with ana that there is certainly no rule that can be assumed, often the treatments are started before the gradeing of stone is started as a mater of course. So much depends on the particular cutting /cooking factory as to there normal procedure.
They only couple of beautiful star sapphire we have found in the last 8 years we gave away for next to nothing to help  pay the end of month bills! The next I will keep!

Cheers Andrew Lane
www.aussiesapphire.com.au

They only couple of beautiful star sapphire we have found in the last 8 years we gave away for next to nothing

oh my!

There are LOTS of synthetic ones around also - made by Linde.

Date: 4/29/2005 9:36:30 PM
Author:widget
I''ve been meaning to ask this for quite a while. Did I just imagine it, or am I right in assuming that most star sapphires and star rubies have NOT under gone heat treatment?

It seems that I read somewhere that it is the rutile inclusions in these stones that cause the star effect...that if a stone were heated, it would lose its star...

Is this correct?

Thanks!

widget

In ruby and sapphire, there are a couple forms of asterism. The most common is created by exsolution of the mineral rutile (TiO2). The best star stones featuring rutile are found in Burma and Sri Lanka. These sources produce star stones mainly in shades of red and blue, from white to intense colors.

In Fe-rich stones from Australia and Thailand, black-star sapphires are found. In these pieces, the silk is not rutile, but hematite (Fe2O3). The stars produced in these stones tend to be less sharp, since the hematite is more platey.

When a rutile-silk bearing corundum is heated to a high temperature, the rutile is partially resorbed into the stone, destroying the star-producing inclusions. Thus a star is a good indication that a corundum has not been heated at a high temperature.

However, there is one caveat. It is possible to use a lattice-diffusion heat treatment to induce a star in ruby and/or sapphire. This star effect exists only at the surface, and is easily detected by a decent gemologist.

And then there are the synthetic (''Linde'') star rubies and sapphire.

In summary, if you can rule out lattice diffusion treated stones, along with synthetics, then the fact that a stone has a distinct star is strong evidence it has not been heat treated at a high temperature.