Natural And Synthetic Yellow/Orange Sapphires

(via ICA Lab Alert, No. 5, December 1987) AIGS writes:

Subject
The detection of color banding/growth zoning in natural and synthetic yellow/orange sapphires.

Method
Color banding, either straight or curved, can be detected much more easily by using a technique developed at AIGS in 1981. This involves the use of a frosted (diffused) blue filter over the microscope’s light source.

When looking for color zoning in yellow sapphires, the usual practice is to immerse the stone in methylene iodide. However, with a yellow stone in a yellow liquid over a yellow (incandescent) light, there is little chance of finding yellow bands of color. Using a white (fluorescent) light helps a bit, but not enough. AIGS have found that by using a frosted blue filter it becomes a much more easier to locate color bands, either straight or curved, as blue is the color being absorbed the most in yellow stones. Sometimes we stack two or three blue filters on top of one another. Although this does not cut down on the light intensity, it still makes it much easier to locate the color zoning. Using the frosted blue filter plus immersion, it is possible to locate straight or curved color banding in about 95% or more of all natural and synthetic yellow/orange sapphires. Furthermore, a green filter can be used for rubies—the color of the should approximate the absorption maxima of the stone.

E Gubelin writes:
To use a frosted blue filter and examine the gem in immersion is an excellent suggestion (though known to experienced gemologist for many years already). The effect may be enhanced if one close the diaphragm to about half or about a quarter of its diameter below the immersion cell.