Fluorescence is the emission of visible light from a substance under the stimulation of radiation of a shorter wavelength. It may be stimulated by (visible) light but most useful are blue light, short wave ultra-violet (UV) light, long wave UV and x-rays. Daylight contains some ultraviolet light and some artificial light sources (i.e. photoflood lamps) put out a considerable amount of UV light.
An example of the effects of such light is blue fluorescing diamonds. Diamonds which fluoresce blue in UV light may have a yellow tint to them in UV free white light which is cancelled by the blue fluorescence in (daylight) conditions.
A strong light (500W) is filtered through a saturated solution of CuSO4 on to a specimen or a light is hone through an appropriate blue filter. When a red filter is used to observe stones in the blue light only red light actually emitted (fluoresced) by the stones under the stimulation of the blue light is observed. Stones that show up red include: ruby, synthetic ruby (verneuil), emerald, synthetic emerald, red spinel, synthetic red spinel, pink topaz, alexandrite.
Used with a spectroscope the following observations/differentiations may be made:
- Corundum (ruby) from red spinel which shows (organ-pipe) lines in the red end.
- Red spinel from synthetic red spinel. The synthetic has a single line in the red (like ruby) and lacks the “organ-pipe” fluorescence lines. To differentiate ruby from synthetic red spinel one looks in the blue where the lines characteristic of corundum are lacking.
Synthetic (verneuil) rubies and synthetic emeralds often show a brighter red than the natural stones. The effect is apparently stronger using an infrared filter to view them with. Where iron is present it acts as a damper on the red fluorescence, i.e. Siam rubies and emeralds from South Africa and India may be almost inert.
Natural black pearls show a dim red glow while dyed ones (using AgNO3) are inert.
Long Wave UV 365.0 nm Similar to crossed filters in some instances.
Natural yellow sapphires fluoresce yellow, synthetic yellow ones are inert. Natural colourless fluoresce orange, synthetic colourless are inert. Synthetic orange sapphire fluoresces red (from chromium).
Yellow-green synthetic spinels fluoresce bright green. Blue synthetic spinels fluoresce red, most naturals do not.
White zircon shows a yellow fluorescence (UV may cause a reversion of colour to original yellow or brown, heat may remedy this).
Fluorspar fluoresces brightly.
Natural emerald is usually inert, most synthetic glows bright red. Ultra long wave UV (4l0.0 nm – 3l0.0 nm) shows a pronounced difference in fluorescence between most natural emeralds (inert) and synthetic emeralds (bright red fluorescence).
Diamond fluoresces in all colours, the fluorescence may be analyzed with a spectroscope in some cases for identification. When cooled with dry ice or liquid nitrogen some diamonds show fluorescence lines at 415.0 nm and 504.0 nm. Irradiated diamonds may show a line at 594.0 nm which is diagnostic. Diamonds that fluoresce bright blue show a yellow phosphorescence when held in cupped hands after the light is turned off – the only blue fluorescing gemstone to do so. Photographs of the pattern and colour of fluorescing diamond set jewellery may serve to identify it for insurance or other purposes.
Short Wave UV, 253.7 nm (NB: is harmful to skin and eyes) Many stones show similar reactions as to long wave.
Synthetic blue sapphires show a greenish or white-blue glow, synthetic white stones a deep blue glow, naturals in both cases are usually inert. Some yellow sapphires fluoresce green at the surface. If suitable precautions are taken synthetics examined with a lens while fluorescing will often show curved structure lines.
Benitoite which resembles sapphire fluoresces bright blue while natural sapphires are inert.
Synthetic rubies and emeralds both often glow brighter red than their natural counterparts. Both are much more transparent to 253.7 nm radiation than the naturals and this forms a test using contact immersion photography or by using apiece of (blue fluorescing) scheelite as an indicator. The suspect stone is placed over a hole in an opaque material with the scheelite below the stone and a short wave UV source above. If the scheelite fluoresces it is receiving light through the suspect stone and it is a synthetic stone. It should be noted that some new synthetics do not pass UV light.
Danburite (1.63 RI) fluoresces bright blue while topaz (1.63 RI) is inert or glows orange and yellow.
Garnet-topped doublets: glass fluoresces yellow (or greenish) and garnet remains dark. In some triplets the cement layer fluoresces brightly providing rapid identification. Many composite stones can be fairly easily identified as such in this manner.
White synthetic spinels glow bluish-white, useful for rapidly picking them out in set jewellery.
Natural amber glows patchily while ambroid may show swirls.
X-rays (Complex lab equipment, health hazard)
Most synthetic rubies and emeralds fluoresce brighter red than most natural ones. Synthetic rubies phosphoresce for ten seconds or so while the natural rubies are inert or nearly so.
Hydrogrossular garnet glows bright orange serving to separate it from idocrase (occurs at times in carving identifications). Colourless, yellow and orange synthetic sapphire may fluoresce red due to chromium.
Fresh water Biwa pearls glow bright yellow.
Synthetic emeralds may phosphoresce a dull red while naturals are inert.
Gilson synthetic emeralds glow bright red.
Cultured pearls show a yellowish fluorescence, natural salt-water pearls are usually inert. Fresh water (natural) pearls fluoresce yellow but only at the surface cultured fresh water pearls glow from within.
Some synthetic corundum manufacturers (Ramaura) have in the past doped their products to allow identification using fluorescence as an indicator. Some of the new hydrothermal and flux-melt products however act very much like natural stones under all stimulation. It is necessary to review the literature to find references to new developments.