Colour

Many colours in various hues often with brownish overtones. Rarely pale blue or colourless. Heat treated stones – Bright pale blue, orange or colourless.

Cause of  Colour

Uranium and Thorium.

Optical Character

 Uniaxial positive.

Pleochroism

 Mostly noticeable to very weak but strong in heat treated blue stones, the o-ray being the deeper shade of colour.

Colour change

No

Spectrum; no polar, calibration in nm.

High type - 691; 683; 662/660; 653; 621; 615; 589; 562; 537; 516; 511;484; 460. Dominant 653 is diagnostic, stronger lines underlined, others weak. Heat treated blue and colourless 653 and 660 possibly others but generally faint.

Ordinary ray

As no polar; but 662;653 and 516 are stronger. Lines at 621 and 615 are missing. Heat treated stones -  any visible lines stronger.

Extraordinary ray

As no polar; but 683;662/660; and 515 are stronger. Lines at 621 and 615 are present. Heat treated stones – Little or no change.

Alpha ray

Not applicable

Beta ray

Not applicable

Gamma ray

Not applicable

Emission lines

No

Database reference

None yet

Notes

Myanmar stones - Strong dark lines. Sri Lankan - Narrower lines but sharp.