Colour

Dark green, greenish blue, dark blue to violet. Rarely a cobalt blue.

Cause of colour

Mainly iron. Low iron plus cobalt contributes to finer blues.

Optical Character

Isotropic                   

Pleochroism

Not applicable

Colour change

Violet stones may show a colour shift to purple in tungsten light.

Spectrum; no polar, calibration in nm.

A strong line at 458 and a weaker one at 478 are variable with iron content. Bands at 555(narrow) 585(broad)and 635(broad) vary in intensity being strong in Co. rich stones where iron lines are weak.

Ordinary ray

Not applicable

Extraordinary ray

Not applicable

Alpha ray

Not applicable

Beta ray

Not applicable

Gamma ray

Not applicable

Emission lines

No

Database reference

GN3;BL!;BL2; and BL3

Notes

Cobaltious spinel BLF0856 is a bright red under a Chelsea Filter.