With regard to the darkening with age, it is most like due to oxidation. Amber, i.e., tree resin, is mainly polymerized carbon and hydrogen. Chemically it is a plastic. The stuff will readily react with oxygen in the air, causing it to craze and opacify.

Think of an old plastic toy or doorknob from the 50's, before chemists started adding antioxidants to the material. The toys or knobs or other fabricated items from this era will literally crack and break in you hand.

It's the same principle.

Many universities do research on amber and copol, Google is full of academic sites. One of them is listed below. Nancy