Colour

Pale to royal blue and dark blue;

Cause of colour

Titanium-Iron transfer charge); Iron (Fe.2+- Ti4+) and Fe3+-Fe3+

Optical Character

Uniaxial Negative    

Pleochroism

Distinct as two shades of blue and  pale or greenish blue

Colour change

Possible in pale blue stones which contain a small amount of Cr.

Spectrum; no polar Calibration in nm.

Pale blues have a weaker absorption with possibly a fluorescence line in the deep red. Saturated blue stones absorb strongly from deep red to yellowish green with a faint line at 450 in the deep blue. Dark blues  may show additional lines at 460 and 471in the blue.which may merge with the 450 as a broad band.

Ordinary ray

Absorption is strongest in this ray.

Extraordinary ray

Absorption is weaker

Alpha ray

Not applicable

Beta ray

Not applicable

Gamma ray

Not applicable

Emission lines

Possibly in pale bluestones which contain Cr.

Database reference

Excel files BL1; BL2 and BL3

Notes

Sri-Lankan pale blues may only show a very weak line at 450: