Natural Yellow Sapphire (Pukhraj)

The yellow color variety of corundum species is known as yellow Sapphire.
The chemical composition of natural yellow sapphire is Al2O3 + Fe. When corundum is pure it is colorless and as the traces of iron come in the structure of corundum it gives yellow color and the variety becomes yellow sapphire.

The natural yellow sapphire is found in many qualities. The quality of a gemstone is judged by its color, clarity, cut and carat. Color- (better the color better the quality), Clarity- (cleanness or free of inclusions, lesser the inclusions better the quality), Cut- (A well proportioned cut is having more value). Carat- (Larger the size higher the value).

The gemstones are valuable and peoples of gem trade are always tried to improve the quality of gemstones by doing some treatments. In case of yellow sapphire there are some treatments done to improve its color.
Following are the treatments done on yellow sapphires.

1. Heating.   2. Beryllium diffusion.   3. Irradiation

1. Heating- The yellow sapphire is heated on a very high temperature of 1500º C to 1800º C in oxidizing condition to improve its color. The high temperature changes the color due to Fe+2/Fe+3 pairs of unstable color centers or traces of Fe+3 and color centers. The color improved by heating is possible to detect and the gemological laboratories are identifies and mentions about the heating in the reports. The color which is improved by heating is always not necessarily permanent. The heated yellow sapphire may lose its color after some time. 

2. Beryllium diffusion- The color of yellow sapphire is modified or enhanced by this treatment. This treatment is done on a very high temperature heating along with beryllium element. The beryllium element is very light and highly reactive on high temperature it diffuses inside the structure of sapphire and brings a noticeable change to its color. The color of beryllium treated yellow sapphire is stable in general and it is possible to detect in most of the cases by laboratories.

3. Irradiation- X-rays or gamma rays are used to improve the color of yellow sapphire. The light color sapphires are kept under the exposure of these rays. X-rays and gamma rays are very strong radiation sources that can make the changes in the internal structure of sapphires and also in other stones to improve or to change the color without any external or internal noticeable effect to the stone. The color produced by this treatment is always not permanent and may fade over a period of time. The gemological laboratories are not able to identify this treatment.

Conclusion- The yellow sapphire is a very controversial stone for jewelers and for the laboratories because of its unstable color. If a gemological laboratory is identifying and describing “Natural yellow sapphire” with a comment “No indications of heating or thermal enhancement” in the report, is good to wear, but there is no guarantee about the color stability. In some cases the color of yellow sapphire may decrease.        

Author

Mustaqeem Khan
Director, IGITL Jaipur


The difference between Lead Glass filled Rubies and Heat treated Rubies
Ruby, Blue sapphire and Yellow Sapphire are the most famous and fascinating stones of corundum family. Ruby is the most valuable color gemstone in the world.    Heat Treatment of Rubies and Sapphires (Corundum) is a Common practice in Sri Lanka and Thailand. The material of good color and clarity is limited and poor quality is frequently available. To make the poor quality more attractive and saleable in the market, Heat treatment and other methods have been applied. The heat Treatment process gives a wonderful change in the color and clarity of corundum and also the changes are stable and permanent. The price value of untreated corundum is more to a similar color and clarity treated corundum. There are various factors for which the heat treatment is done on corundum.

• To lighten the over colored stones. (Deep blue to medium blue).
• To darken the lighter color stones. (Light blue to medium blue, light yellow to yellow).
• To remove the secondary shades. (Purplish color from rubies).
• To increase the transparency of stone by diluting silk inclusion into the structure of corundum.
• To develop star effect in stone. (Light blue or medium blue sapphires becomes star sapphires.
• Diffusion of blue or red color in colorless or light color sapphires.