PRESIDIUM Synthetic Ruby Identifier User Handbook Manual page 28

Low Reading
The first 4 bars from the left fall within the Low reading range and if
any measurements within this range is recorded, a blue light will light
up within these 4 bars (Fig 3.2).
Low High
Low High Start Test
UV Transmittance
UV Transmittance
Fig.3.2 Low Reading
A Low reading with blue light means the UV light transmittance
ability of the tested gemstone is low and this is probably due to trace
elements within the stone which can occur during the natural ruby
growth process or through flux and hydrothermal synthetic ruby
growth processes.
You are advised to conduct further supporting tests to ascertain if your
tested gemstone is a natural ruby.
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