Sapphire inclusion rainbow

[ephsea]

I have a heated Montana sapphire with inclusions that make rainbows. Most of the time it just gives the gem the fire of a more dispersive gem, but in the right light you can actually see a rainbow inside the gem. I finally got a blurry shot that shows it a bit. Does anyone know what type/types of inclusions would cause this? At the top of the gem you can just make it out.

 

[Michael_E]

Rainbows are caused by fracture surfaces where those surfaces are just the right distance apart so that the reflected light from both surfaces "interferes" with itself. Sort of like a diffraction grating.