Colour

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

Cause of  Colour

Uranium,  Thorium and rare earths may be present.

Optical Character

 Uniaxial positive.

Pleochroism

 Mostly noticeable to very weak but strong in deep blue and heat treated blue stones.

Colour change

Heat treated purplish blue stones appear bluish green in tungsten.

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 pale blue and colourless stones -  653 and 660 possibly others but generally faint.

Ordinary ray

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

Extraordinary ray

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

Alpha ray

Not applicable

Beta ray

Not applicable

Gamma ray

Not applicable

Emission lines

No

Database reference

See all Excel files

Notes

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