Popular Science, heinäkuu 1922
In the heart of a genuine pearl lies a mysterious,unseen quality that gives the gem its warm beauty and its value. What is it? How can this quality be detected so as to distinguish positively between the real pearl and the imitation?
One way has been discovered - by the strange, invisible rays of ultra-violet light.
The unseen is detected by the invisible. A pearl to be tested is held in the palm of the hand beneath the rays of a strong, bluish light. if it is a true gem, it seems to glow and burn as though it were painted with phosphorus; while if it is artificial, the pearls appears to be dull black.
To produce the peculiar effect, a light, rich in ultra-violet rays of long wave length, is required. Recently a small, compact mercury.vapor light has been perfected. The lens of this lamp is made of quartz, which does not absorb ultra-violet rays. if ordinary glass were used, all pearls would look alike under the light.
A patent color filter made of a nickel-oxide glass is mounted over the quartz lens. This glass absorbs almost all the visible light, while it permits the longer ultra-violet rays to pass. if you look into the light when this filter is in place, your eyes seem to be bathed with an invisible, balmy fluid. This sensation is the result of fluorescence of your eyeballs. Under the invisible light, your eyes shine like a cat's, and your teeth and finger nails glow; yet a porcelain filling in your mouth will look black as coal.
It is this property - of causing fluorescence in some objects, but not in others - that gives ultra-volet rays their value in testing pearls.
The testing cabinet is a box about three feet long. 16 inches high, and 16 inches broad. One end is open, except for a tight-fitting black curtain. The other end is pierced for the insertion of the lamp, lens, and color filter. in the top of the box is a small slot, fitted with a mask.
To test a suspected pearl, the jeweler thrusts his hand, holding the gem, through the curtain, and looks down into the miniature darkroom. if the pearl is genuine, it will fluoresce with a soft, bluish glow.
It is said that many manufactured gems, masquerading as the real thing, can be detected by this method.