Notes From The Laboratory

2007: Here is an excellent insight to identification (diamond) from my mentor.

(via The Journal of Gemmology, Vol.XIX, N0.2, April 1984) Kenneth Scarratt writes:

It is not unusual for a group of gemologists to disagree over the color of gemstone. Some colored diamonds in particular tend to possess colors which are most difficult to describe. However, when you see a diamond actually change color from one distinct color to another before your eyes it can shake any confidence you may have in your own eyesight.

Such was the case when late one afternoon I decided to make start on identifying the nature (natural or treated) of the color of 2.02 ct brilliant-cut diamond. All I really had time for was to make out my worksheet, giving a full description of the stone, and a short microscopic examination before it had to go into the safe for the night. On the worksheet I stated in a most positive fashion that the color of the stone was green. The next morning when the safe was opened I immediately retrieved the envelope containing the diamond, took it to a work bench and removed the stone from it. There before me lay a brilliant yellow stone. After checking the envelope to make sure that it was the one I put in the safe the night before (it was) I decided to check the stone’s weight against my record, but as I picked it up to take it to the balance its color started to change through various shades of yellow and yellow/green until it was back to the color it was the night before.

These so-called ‘chameleon diamonds’ have been reported upon before, and the change has been variously described as being associated with changes in temperature or in the amount of light reaching the stone. A manufacturer would notice the effect because it is said that these stones glow red on ‘the wheel’ and change to yellow shortly afterwards, from which they return to their normal green at room temperature; whereas a trader might become aware of the type of stone he had in a similar manner to that in which I had become aware of the peculiarities of this stone.

The color change from green to yellow, unless one includes the slight cooling which may occur if the stone is placed in a safe overnight, is usually described as being dependant upon a temperature increase, such as placing the stone on a hot plate, rather than a decrease; and so it was interesting to discover that when we reduced the temperature of this stone to at first 120K in the laboratory and then to 77K at King’s College, London, whilst recording the spectra, the color of this stone once again became a brilliant yellow.

The differences between the room and low temperature spectra are quite evident. The general appearance of the spectrum at room temperature is approaching that of a normal Type 1b with a weak 415 (Type 1a) peering out of the gloom and an unusual absorption hump covering the yellow, orange, red and N.I.R—the are of greatest transmission being in the green. At the lower temperatures there is clearly a sharpening up of the 415, but more importantly there is a lessening of the absorption hump in the red, orange and yellow, allowing the stone to transmit to a greater extent in this region as well as in the green, thus resulting in a yellow stone.

One assumes that changes of a similar nature may take place when the stone is heated; however, we restricted ourselves to room and low temperature spectroscopy only. The luminescence effects produced by this tone were—long wave ultraviolet, a very strong bright yellow followed by a very strong greenish phosphorescence; short wave ultraviolet, a strong and bright yellow/green followed by a very strong greenish phosphorescence; and X-rays, a blue/green followed by a strong green phosphorescence.