Brown Tourmaline? rainbow 400x magnification.

[aussiefossils84]

Hi I have had this tourmaline for many years from Anakie region qld. I have recently sliced a specimen off the hardest part of rock and shows blotchy orange spots through dark stone if looking 100x through microscope, I would like to know why it has a rainbow halo on outer edge of stone when 400x magnification?
It also has straight rainbow lines running through stone at 400x is this what I am meant to see on tourmaline?

Weighs 280 grams 6cm high approx. 5cm across bottom.

 

[aussiefossils84]

Also it is not at all magnetic.