Odd stain in bottle...what causes this?

[jughed]

Hi everyone, I'm new to this site, and from what I have read so far, there are many knowledgable collectors here. I primarily collect local dairy and soda bottles, and any oddities that I may dig up.
One little amber bottle that I recovered when digging around the old city dump site has odd staining. I'm pretty sure that it occurred when the bottle was made because the stain is between the inner and outer surfaces. So far I can't find any info on what may cause this. Any help is appreciated.

 

[woody]

Some of my beliefs of what may have caused the staining are:
1. The glass has absorbed some type of chemical to stain the color.
2. It may have been subjected to fire.
3. Something may have been in the gather of glass when the bottle was made.
4. A chemical reaction to the type of soil it was buried in.