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Why are stones treated/heated? How is the decision made?

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haagen_dazs

Brilliant_Rock
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Hi all
My question sounds simple but I wanted a more comprehensive answer (if there is)

Why are stones heated and treated?
I know that this basically enhances and improve the colour of the gem stones.
Are there any other good reasons why they are treated?

Since untreated stones sell for about 2-3x (am i right?) the cost on heated /treated stones, so the miner could sell the stones untreated to get more money right?
Unless the stones are of really poor colour quality, I can understand why they would be treated and enhanced.

This brings me to the next point.
What happens if a fantastic colour gemstone is found?
Do they generally leave it untreated?
What happens if they still choose to treat the gemstone? Will the colour become doubly or triply fantastic?

Are enhanced gemstones treated before or after cutting the rough?

Can treatment lead to a colour detrimental effect to the gemstone?
 

morecarats

Shiny_Rock
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Date: 10/23/2009 12:48:52 AM
Author:haagen_dazs
Hi all
My question sounds simple but I wanted a more comprehensive answer (if there is)

Why are stones heated and treated?
I know that this basically enhances and improve the colour of the gem stones.
Are there any other good reasons why they are treated?

Since untreated stones sell for about 2-3x (am i right?) the cost on heated /treated stones, so the miner could sell the stones untreated to get more money right?
Unless the stones are of really poor colour quality, I can understand why they would be treated and enhanced.

This brings me to the next point.
What happens if a fantastic colour gemstone is found?
Do they generally leave it untreated?
What happens if they still choose to treat the gemstone? Will the colour become doubly or triply fantastic?

Are enhanced gemstones treated before or after cutting the rough?

Can treatment lead to a colour detrimental effect to the gemstone?
Mark, this is a very large topic, since there are many kinds of gem treatments (such as heating, diffusion, irradiation, fracture-filling, dyeing and coating).

To take the case of simple heat treatment for sapphire, generally the color of the unheated stone is unacceptable or mediocre, so the stone has little value in the market. Heat treatment at high temperature usually affects impurities in the corundum, with the result that the clarity is enhanced and sometimes the color.

If a natural sapphire with good color and clarity is found, it is usually not heated, since in most cases its appearance will not be improved by heating. Its value as an untreated sapphire will be higher.

Most gemstones that are heat-treated are usually heated before they are cut and polished.

Heat treatment of corundum at high temperature is not believed to be detrimental to the durability of the gem. But I don''t think this question has been definitively answered, so you may find opposing views on this.
 

haagen_dazs

Brilliant_Rock
Joined
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Messages
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Date: 10/23/2009 2:50:01 AM
Author: morecarats
Date: 10/23/2009 12:48:52 AM

Author:haagen_dazs

Hi all

My question sounds simple but I wanted a more comprehensive answer (if there is)


Why are stones heated and treated?

I know that this basically enhances and improve the colour of the gem stones.

Are there any other good reasons why they are treated?


Since untreated stones sell for about 2-3x (am i right?) the cost on heated /treated stones, so the miner could sell the stones untreated to get more money right?

Unless the stones are of really poor colour quality, I can understand why they would be treated and enhanced.


This brings me to the next point.

What happens if a fantastic colour gemstone is found?

Do they generally leave it untreated?

What happens if they still choose to treat the gemstone? Will the colour become doubly or triply fantastic?


Are enhanced gemstones treated before or after cutting the rough?


Can treatment lead to a colour detrimental effect to the gemstone?

Mark, this is a very large topic, since there are many kinds of gem treatments (such as heating, diffusion, irradiation, fracture-filling, dyeing and coating).


To take the case of simple heat treatment for sapphire, generally the color of the unheated stone is unacceptable or mediocre, so the stone has little value in the market. Heat treatment at high temperature usually affects impurities in the corundum, with the result that the clarity is enhanced and sometimes the color.


If a natural sapphire with good color and clarity is found, it is usually not heated, since in most cases its appearance will not be improved by heating. Its value as an untreated sapphire will be higher.


Most gemstones that are heat-treated are usually heated before they are cut and polished.


Heat treatment of corundum at high temperature is not believed to be detrimental to the durability of the gem. But I don''t think this question has been definitively answered, so you may find opposing views on this.

thanks!
i know its a big topic but you have answered my questions with enough detail
as i read through literature, i will definitely learn more

Curious one other question

Can certain high temperatures / heat treatments worsen the colour in a gem stone ?? (even though heat treatment is suppose to improve the colour. I am wondering where odd behaviours happen)
 

chrono

Super_Ideal_Rock
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There are many stones that look quite ugly if unheated. Their natural colour is brown, has undesirable gray or some other unattractive undertone. Heating will improve the colour significantly in this case, which of course then increases their value many times over. If the colour out of the ground is attractive, it is left in its natural state. Some stones’ appearance are not affected by heating such as spinel and garnets and as such, they are not heated.

Most stones are heated, then cut. A few like the copper bearing tourmalines are cut first, then heated.

It has not been proven that heating affects the structural integrity of the stone. Most heating is permanent and the colour remains stable.
 

chrono

Super_Ideal_Rock
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You are correct. For different gemstones, there is an ideal temperature that it requires to cook to the right colour. Too low and there might be no effect. Too high and it might overcook to the wrong colour or become too brittle. If I am not mistaken, the cooling process is also important.
 

LD

Super_Ideal_Rock
Premium
Joined
Jun 29, 2008
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10,224
There are a number of gemstones that are routinely heated. Zoisite (Tanzanite) is one such gem. When it comes out of the ground it can be light blue, brown, grey but after heating it becomes a beautiful velvety royal blue/purple. Some heat to a much richer colour than others.

However, occasionally, Tanzanite comes out of the ground pink, orange, yellow, green etc., and those one are normally left unheated if they''re pretty enough. They can command prices that a great heated blue Tanzanite retails for.

In terms of treatments, if you look at Rubies, if they''ve been BE diffused, they will more often than not have a good clarity and be very red in colour. The lead filled rubies can be more opaque looking (although that''s a very wide generalisation).
 

haagen_dazs

Brilliant_Rock
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Messages
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Date: 10/23/2009 8:31:17 AM
Author: Chrono

It has not been proven that heating affects the structural integrity of the stone. Most heating is permanent and the colour remains stable.


maybe this is a stupid question

can the gem stone colour change over time? (untreated)

lattic breakdown or change in molecular stucture or oxidation or reduction or rearrangement of atomic structure or etc etc ?

You mentioned "most heating" so that implies some treatment is not permanent?
which ones would that be?
 

chrono

Super_Ideal_Rock
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Yes. Some gems are known to be night stones where exposure to UV or sunlight will cause it to fade into a colourless stone over time (some quicker than others). Kunzite is one that comes to mind.

Yes. Heating changes the internal structure of the stone. It may also change the crystal inclusions within the gem, causing them to melt or explode. There are usually tell tale signs of heat treated stones usually seen in the form of small fractures or decrepitation feather within the stone around natural mineral inclusions. Note that these are not eye visible, probably not even loupe visible. The temperature used varies depending on the material and the colour to be achieved. Sapphires typically require high temperatures up to 2000 degrees C. Zircon is heated at around 1000 degrees C.

Yes. Most heating is permanent. Kunzite and some yellow sapphires will fade when exposed to strong sunlight over time. Note that it’s not a matter of days but months and years. Zircon can be unstable, so after heat treatment, they are exposed to sunlight for several days, then stored in the dark for up to a year to remove the unstable ones.
 
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