Colour

Greyish  blue with a purple overtone and red flashes

Cause of colour

Vanadium. Chromium may also be present.

Optical Character

Uniaxial negative     

Pleochroism

Daylight;  ω -  Pale purplish blue; ε - Pale greyish green            Tungsten; ω -  Pale purplish pink; ε – Pale orangy yellow; 

Colour change

Greyish blue in daylight to strong purplish red in tungsten light

Spectrum; no polar, calibration in nm.

Absorption band in green centred at 570. is responsible for colour change and dichroism Narrow line at 475 in blue due to vanadium.

Ordinary ray

Absorption band in green strengthens and widens.Fluorescent lines may be seen in red due to chromium.

Extraordinary ray

Absorption band in green weaker and narrower.

Alpha ray

Not applicable

Beta ray

Not applicable

Gamma ray

Not applicable

Emission lines

Fluorescent doublet may be seen at 693/694nm. if  Cr. is  present

Database reference

Blue Gems. Excel file BL3

Notes

Compare to alexandrite