I have a sample of Mercury in a plastic vial, The vial is a BD Vacutainer (genuine), made of polyethylene terephthalate. It is used for blood samples, and the one I used was new.
When the vial was new, it was almost as transparent as glass, as seen in the image:
However, after about a month, here is what the vial looks like:
As you can see, the vial has darkened.
The vial was sealed with a small initial layer of hot glue about 1 centimeter from the top, then the last centimeter was filled to the brim with epoxy, to ensure it won't leak.
I have been handling the vial quite a lot, but it has been stored in the padded box as seen in the image.
What is causing the plastic to darken? Once again the vial is made out of polyethylene terephthalate if that could be useful.