Tell you what fella. Money and trinkets mean nothing. Its her 18th. The best birthday present you can give her is "an experience". Put it this way. In 20 years time she wont look back with fond memories on the shiny gold necklace you bought her. But she would look back with fond memories on the fantastic day she had.
By all means but her something nice (sentimental) on her birthday. But put your money and efforts onto making it a day/event she will remember.
Take the day off (both of you) spend it together doing things she enjoys. Matron!
Lots of WIN in this, trust me.