Gem Testing

(via The Journal of Gemmology, Vol.10, No.1, January 1966) A E Farn writes:

Recently, a rather rubbed brown cabochon stone in a ring with a reasonable ray to the stone came in for test. At first glance the stone looked a quartz cat’s eye, by the coarse nature of the ray. However, as the stone was worn by bad usage, it could be partially the reason for a poor chrysoberyl looking like quartz. The stone was backed; this prevented one looking at the back of the stone for a hint of quality.

However, the very useful distant vision reading methods of taking a refractive index soon solves the question of quartz or chrysoberyl cat’s eye. Maybe I am going a long way round to bring the point home, but the telephone rang whilst I was looking at the stone in question, and having dealt with that matter I returned to the stone, put a spot of liquid on the refractometer and took a spot reading. I saw quite a reasonable changeover light bar at 1.74 which seemed reasonable enough—completely divorcing it from quartz. Automatically, I turned the spot intensity lamp on and tried to see the chrysoberyl absorption spectrum and could not. I was not surprised; there was a lot of glare from a reflected light (the stone was backed). Something did not seem quite right, so I took the distant vision again and got a good quartz reading.

Then the penny dropped—after answering the telephone I pulled the refractomete towards me and put on methylene iodide as a contact liquid (I have two dropping bottles and two refractometers). The methylene iodide gave a good spot pattern for itself and the quartz being rubbed it did not react as strongly as it should.

There seems to be some sort of moral here about keeping bottles separate, but actually at the moment of writing we are threatened with a telephone strike at night. Well, all I say is, let us have it by the day and get our testing done without interruptions.