Yellow And Orange Sapphires

(via ICA Lab Alert, No.1, June 2, 1987) AIGS writes:

Background
In 1981, we at AIGS were asked to identify what was undoubtedly among the first heat treated Sri Lankan yellows to come out of ovens of Bangkok. We were told that the stone was treated by an unknown process (which was later found to be heat), and were asked to determine the color stability. This we proceeded to do by performing our usual fade test. This consisted of exposing the stone to heat and light at 1cm distance of a 150 watt spotlight for up to one hour. We were truly surprised when, after a few minutes exposure, the color had become much darker and more brownish (stones treated with irradiation would fade; some untreated Sri Lankan yellows will fade, but in most the color will not change). This change was temporary only; as the stone cooled to room temperature the color returned to normal.

Since this time we have tested over a thousand Sri Lankan yellow/orange sapphires and have found that all of the heat treated yellow to orange stones react in this way. Thus, a simple test for detection of heat treatment in Sri Lankan yellow/orange sapphires is possible.

The test
Place the stone in close proximetry to a source of mild source of heat, such as an incandescent bulb.

Results
Heat treated Sri Lankan yellow/orange sapphire —The color darkens temporarily, becoming more brownish. The deeper the original color, the greater the change. As the stone cools, the color returns to its original state. Control stones should be used so as to detect even slight changes in color.

Irradiated Sri Lankan yellow/orange sapphire —The color will fade, usually within one hour.
Untreated Sri Lankan yellow/orange sapphire —Generally no change, however some stones may show some fading.
Thai/Australian yellow/orange sapphire, heat treated or untreated—no change has been observed in the color of these stones.

(To: Mr N Horiuchi; Subject: Response to comment on Lab Alert No.1) AIGS writes:

Discussion
Mr N Horiuchi has commented on the test we previously described in Lab Alert No.1 to detect heat treatment in Sri Lankan yellow/orange sapphires. Mr Horiuchi stated that this color also did not fade under the same condition as reported on (by) AIGS.

From the above statement it appears that Mr Horiuchi has not understood the text of Lab Alert No.1. As others may also have misunderstood the text, we will describe the test again below.

Heat treatment in Sri Lankan yellow to orange sapphires may be detected by applying a simple fade test. (Caution: This test only works for Sri Lankan stones). Once a yellow/orange sapphire has been identified as definitely originating from Sri Lanka, its color is tested by applying a simple fade test. The stone in question should be placed on a glass (or other nonflammable) platform within ½ cm of a hot 150 watt (or more) spotlight. The idea is to expose the stone to lots of light and heat. After about 15 minutes exposure (as the stone heats up), the color of a heat treated yellow/orange sapphire will have been found to have become slightly darker and more brownish (the deeper the starting color before the test, the deeper and more brownish the color after heating up). This change is temporary only. As the stone cools its color will fade back to the color before the test was started, not, we repeat, not back to the color before the stone was heat treated (by someone else presumably). We believe that is where Mr Horiuchi misunderstood the original Lab Alert.

Other possible reactions
If the stone has been irradiated (either by nature or by man) the color will fade, usually within one hour’s exposure. In most natural Sri Lankan sapphires, however, the color will show no change. Natural yellow/orange sapphires from other sources and synthetic yellow/orange sapphires also show no change.

To make this test more accurate, control stones of similar color to the stone being tested should be used. Then after the stone in question heats up it can be compared to the color of the control stone. In the case of heat treated yellow/orange Sri Lankan sapphires, the change is not subtle in deeply colored stones, and anyone with normal vision should easily detect it, but the comparison must be made quickly before the stone tested cools down.

Dr Kurt Nassau has informed us that under certain conditions, yellow/orange sapphires may get darker upon exposure to some kinds of visible light. We have absolutely no information on exactly what kinds of stones do this or under what conditions. However he has promised us that the subject will be described in detail soon in an article he has written for Gems & Gemology. We have also written an article on the subjects covered in Lab Alerts Nos. 1 and 2 and submitted it to Gems & Gemology. We don’t know when it will appear (or if it will appear). We have had no reply of any kind, even though we sent it 8 weeks ago.

The subject of color in yellow sapphires is extremely complex. We have no illusions that the above information is the last word on the subject. However, over the past ten years we have tested thousands of pieces of yellow/orange sapphire from all sources and this is what we have found. If anyone else could she additional light on the subject, we would love to hear from them.

Dr K Schmetzer replies:
A. Fe³+ or by Fe³+ and Ti³+: Type 1, originating from Nigeria, Thailand, Australia or
B. By a yellow color center: Type 2, originating from Sri Lanka.

By heat or irradiation heat treatment, yellow stones with similar color centers, i.e with absorption spectrum similar to the spectrum of Type 2, but with different stabilities to light or heat are produced.

C. Irradiation treatment, color center: Type 3
D. Heat treatment, color center: Type 4

According to my experience and knowledge, AIGS describes a test for Type 4 stones, and N Horiuchi is dealing with Type 3 stones. Dr Nassau describes Type 2 stones in Alert No.9, and this type of yellow color center may be connected with natural irradiation. The reason for the higher stability of this naturally irradiated yellow compared to artificially irradiated yellow is an unknown but similar results were found by myself with natural irradiated yellow quartz (citrine) and artificially irradiated yellow quartz.