Colour

Pale greenish yellow to shades of green, greenish brown and brown.

Cause of colour

Iron; Small traces of chromium may contribute to some green stones

Optical Character

Biaxial positive         

Pleochroism

Often  just two shades of body colour detected; α -  darker;  γ - paler

Colour change

No, but some green stones may appear a more brownish green in tungsten light.

Spectrum; no polar, calibration in nm.

Main band due to iron centred at 444nm.easily resolved in yellows but more difficult in browns and may cut off in dark greens at 450.Weaker lines may be seen in darker browns at 485 and 505.

Ordinary ray

Not applicable

Extraordinary ray

Not applicable

Alpha ray

444 band and others are stronger and broader in this direction

Beta ray

Untested  - awaiting a suitable single crystal for observation.

Gamma ray

444 band narrower. Weak Cr. line at 680 in some green stones

Emission lines

No

Database reference

None yet

Notes

Some greens may also show a very weak line at 680 due to Cr. and absorb strongly in the deep blue to almost mask the 440 iron band.  No definite colour change is seen but these stones may appear red under crossed colour filters.  - In my opinion these are not Alexandrite