Duncan Miller

Cut rubies, red garnets and red glass can look very similar. There are several techniques that can be used to determine if a red stone is a ruby. These include a semi-destructive relative hardness test (ruby will scratch garnet and glass, but not the other way around); using a polariscope to test for birefringence (ruby is birefringent whereas glass and most garnet are not); and using a dichroscope to see the two pleochroic shades of red in ruby (which are absent in garnet and glass). Microscopic examination may show round bubbles and swirl marks in glass, or inclusions characteristic of ruby and garnet, although often these look very similar.

A simple optical test that depends on the fluorescence of ruby is the so-called ‘crossed filters test’. This was described as a gemmological technique in 1953 by Anderson, but it was first demonstrated a hundred years earlier by Stokes (Webster, R. 1994. Gems: 82, 850-1. Oxford: Elsevier.). It involves passing white light through a blue filter that passes only blue light, and illuminating a ruby only with that blue light. The ruby will reflect some of the blue light, but also will absorb some and emit red light by fluorescence. If the ruby is viewed through a red filter that removes all the reflected blue light it will appear to glow red. Indeed, this happens in daylight anyway, which is why good rubies exhibit a true glow due to their absorbing blue wavelengths of light and emitting visible red light by fluorescence.

Distinguishing red fluorescent ruby from other non-fluorescent red stones simply involves viewing the stones together under ‘crossed filters’ and seeing which appear to glow red. A dramatic demonstration of this involves shining the light from a slide projector through a round-bottomed flask of concentrated copper sulphate solution. The flask acts as a lens to focus a beam of blue light onto the stones. If this is done in the dark, garnet appears black, while ruby glows red. A simpler set-up consists of a small cardboard box – I used an old card file box – with a blue camera lens filter set in the hinged top and a red lens filter mounted in one side as a viewing port. The stones to be tested are placed inside the box on a black background and a strong light shone though the blue filter on top. Viewed through the red filter in the side, rubies glow red while the other stones appear black. It can be illuminated easily with a small torch, and is more portable than lugging around an old slide projector and a flask of copper sulphate solution!

Figure 1: The small filing box is fitted with a blue filter in the opened lid and a red filter in the side

Figure 2: The three red unknown stones are placed on black paper inside the opened box

Figure 3: The closed box, with the three red stones inside, is illuminated by a torch from above, through the blue filter

Figure 4: The three stones, illuminated with blue light through the blue filter in the lid, are viewed through the red filter mounted in the side of the box

Only the central stone is visible, fluorescing bright red. This is a synthetic ruby. The two stones flanking it are red glass and a garnet. They don’t fluoresce, so in blue light they appear black through the red filter.

Another UV gem tip: You can use a long-wave ultraviolet torch to reveal fracture filling with oil in emeralds because the oil fluoresces, just like the oil inclusions in some quartz crystals: http://www.gahk.org/journal/2020/a12.pdf (This downloadable file has been tested and is clean.)