What should I do next? Big argument with my mum?

I guess this isn't usually something to get so upset about, but I can for sure understand the whole inconsideration of things that mean something to you. I'll bet she doesn't seem to care cos she really doesn't understand how much it means to you. Just explain to her that it holds memories and that it'd be greatly appreciated if she gave it back. Try to be mature as possible, and you might end up getting what you want.