Beware if you are having the small 5 year valid non photo green NIE card as a European passsport citizen. It will already completely fade after a few months and you really need it with various companies and official entities.
Do not put it in your wallet.
If it fades too much you will have no proof of the NIE when they ask to have one and copy it and you will have to go through the whole costly and time wasting NIE procedure again, having to provide all kinds of evidence again.
Not sure if one is allowed tp plastify it. Maybe that could protect it better?