You need to explain to her the value of this jewelry in a way she can understand. For example, say, "This necklace and set of earrings cost as much as 50 books! (or 20 other kinds of necklaces, or a Wii system, etc...something she can relate to and values) and explain to her that if she doesn't take care of it, she won't have it anymore.
That's one lesson my parents ingrained in me at a very young age...if you take care of your things, they'll last. Needless to say, because I learned it so young, I still have some cherished things from my youth...books, a few favorite toys, some clothes, etc...all in great condition. I have all the jewelry I've ever had, because I had a jewelry box to keep them in and that was the ONLY place they were ever stored.
Maybe get her a nice jewelry box and a fake set to "play" with...the real set to wear on special occasions.