hi
1st Oct 2013 16:48 UTCAntonios Karatosios
Hi,
We are muddling up different thoughts.
- Yes, virtually all tanzanite is heat treated. That said, I was fortunate enough to pick up an 11 ct ct untreated at auction recently as a bargain (the vendor and auctioneer had not spotted that it was untreated. Amazing. A once-in-10 year real coup!
- My point was that I think it more important to be able to tell tanzanite (heated), blue spinel and and kyanite from sapphire than it is to fuss overly about whether and how a sapphire has been heat treated. As said, heating (and/or pressure) is the way by which all sapphire becomes blue.That said, there are many with loud voices who do not think as I do.
- Ruby/sapphire and relative safety in buying. Hard to make a good comparison, As said, mid-range priced sapphire should be expected to be heated as a matter of routine and (like tanzanite) only outstanding specimens avoiding this treatment. This treatment is known to be over 2,000 years old. However, good-ish ruby is only sometimes heat treated
- to dissolve some inclusions, especially rutile, and thus to improve the clarity and value of the pieces. Heat plays no part in the way ruby (or pink sapphire) come to have its colour. However, in the last 20-30 years particularly, a large quantity of very poor quality ruby, unsaleable because of cracks, fissures and other cavities is being 'healed' by being filled with lead glass before being finally cut and polished. This produced a saleable item though there can be more glass than ruby in a specimen. Cr*p quality sapphire is glass-filled .in the same way. So, you need to be careful, though for slightly different reasons, which ever you are buying.
The picture.... This is glass 99+% coloured by cobalt. - and very pretty it is when worn - but, like all glass it scratches and chips easily and does not go on looking good for more than a few wearings. For the telltale, look at the five facets closest to the camera that are relecting light falling on them, partly acting as mirrors to reflect white incident light to the camera's lens. Look carefully, and you can see faint blue speckles.These are surface reaching particles of the cobalt colouring compound that are exposed on the polished surface. These spots to not reflect the white incident light and just show as blue spots. Here's a higher magnification shot that makes the point absolutely clear that this specimen needs more checking.
Turn it over and still examining through the microscope, and now using what is called 'dark-field' lighting from below the stone, it is easy to see that it contains bubbles (not ever found in naturally formed crystals). there is one main at 12 o'clock from the centre of the table and a whole squadron of them at one o'clock near the girdle. Thou can also see traces of polishing lines left on pavilion facets of the soft glass.
And here's a closeup of the main bubble towards the centre of the table, a 'copy-book' example of an air bubble in glass.
My point here is that too many retail customers (and some trade buyers too) expend too much energy on discussing the evil of treatments that they and in some cases no one can detect. Time better spent in learning to recognise at a glance, with no more than a pocket light and a loupe, many imitations (fakes) and lousy quality material being passed held together with glass or plastic.
We are muddling up different thoughts.
- Yes, virtually all tanzanite is heat treated. That said, I was fortunate enough to pick up an 11 ct ct untreated at auction recently as a bargain (the vendor and auctioneer had not spotted that it was untreated. Amazing. A once-in-10 year real coup!
- My point was that I think it more important to be able to tell tanzanite (heated), blue spinel and and kyanite from sapphire than it is to fuss overly about whether and how a sapphire has been heat treated. As said, heating (and/or pressure) is the way by which all sapphire becomes blue.That said, there are many with loud voices who do not think as I do.
- Ruby/sapphire and relative safety in buying. Hard to make a good comparison, As said, mid-range priced sapphire should be expected to be heated as a matter of routine and (like tanzanite) only outstanding specimens avoiding this treatment. This treatment is known to be over 2,000 years old. However, good-ish ruby is only sometimes heat treated
- to dissolve some inclusions, especially rutile, and thus to improve the clarity and value of the pieces. Heat plays no part in the way ruby (or pink sapphire) come to have its colour. However, in the last 20-30 years particularly, a large quantity of very poor quality ruby, unsaleable because of cracks, fissures and other cavities is being 'healed' by being filled with lead glass before being finally cut and polished. This produced a saleable item though there can be more glass than ruby in a specimen. Cr*p quality sapphire is glass-filled .in the same way. So, you need to be careful, though for slightly different reasons, which ever you are buying.
The picture.... This is glass 99+% coloured by cobalt. - and very pretty it is when worn - but, like all glass it scratches and chips easily and does not go on looking good for more than a few wearings. For the telltale, look at the five facets closest to the camera that are relecting light falling on them, partly acting as mirrors to reflect white incident light to the camera's lens. Look carefully, and you can see faint blue speckles.These are surface reaching particles of the cobalt colouring compound that are exposed on the polished surface. These spots to not reflect the white incident light and just show as blue spots. Here's a higher magnification shot that makes the point absolutely clear that this specimen needs more checking.
Turn it over and still examining through the microscope, and now using what is called 'dark-field' lighting from below the stone, it is easy to see that it contains bubbles (not ever found in naturally formed crystals). there is one main at 12 o'clock from the centre of the table and a whole squadron of them at one o'clock near the girdle. Thou can also see traces of polishing lines left on pavilion facets of the soft glass.
And here's a closeup of the main bubble towards the centre of the table, a 'copy-book' example of an air bubble in glass.
My point here is that too many retail customers (and some trade buyers too) expend too much energy on discussing the evil of treatments that they and in some cases no one can detect. Time better spent in learning to recognise at a glance, with no more than a pocket light and a loupe, many imitations (fakes) and lousy quality material being passed held together with glass or plastic.
