Hmmm...what about re-presenting a gift you MADE for her when you were a child? The sentiment is the same but you could include a card and write something saying to the effect of "Even though I made this for you when I was 7, I loved you then and I still love you now. You will always be my MOMMY!"or something saying how much your love has grown from childhood to adulthood.
If that doesn't work, try diamonds.

j/k
But, seriously what does Mom like? Her interests, hobbies? Does she like theatre? get her tickets to the next play or performance. Ya see where I'm goin' with this? Take your ideas from there. Good luck!!