So we have weapons racks and cabinets that prevent weapons/armour and clothing from decaying. Shouldn't there be a jewelry box or chest or something that would do the same for items of jewelry?

There is one! Just ask your friendly Artisan, whomever that may be.

There are boxes, and some designs are described as jewelry boxes, but they do not preserve jewelry stored in them, which I think was the point of Tandrin's suggestion. All they are is boxes which hold four things and have no other powers.

I've tried to get this idea (and a few other Artisan changes) envoyed, but it's impossible to get tradeskill ideas through the envoy process, there's never room for them.