Over time, the amber oxidizes (reacting with atmospheric oxygen) - this process is also called "crazing". The oxidation usually takes several years before you realize it and when the damage occurred it is in every way too late to save and restore the damage (The oxidized layer can be ground away, but in many cases it is not an option). Because of the slow process and lack of knowledge, there are many collectors who neglect the importance of protecting their amber collection.
I have seen several one-in-a-million piece, which has a value of several thousand dollars and irreplaceable scientific value, been stored incorrectly and therefore oxidized very bad - their value is in every way fallen to zero!
The amber surface will be darker (red) than the inside and small cracks will occur in the surface (like on dry clay), allowing oxygen to penetrate into the piece - worst of all, this will also dissolve" and darken the inclusions.
The inclusions in this piece of Baltic amber (12 mites and 1 isopod) are exposed to the undesirable effects of oxidation. The small inclusions will slowly "disappear" and the large progressively become black. Once this process has started it cannot be stopped directly or restored, but only slowed down. Note that you can see in the picture, how the oxidation penetrates the piece.
Sun light (UV-light)
There are several ways to effectively slow the process down. Some of the ways are (obviously) more effective than others - but in the end we do not know their long-term sustainability. We know the theoretical durability and we have some knowledge from different experiments, but in many cases there are several unknown factors which have an impact of many years!
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