I finally decided to narrow it down to two options. The necklace that Rebecca kindly let me borrow and a necklace that my Mum lent me that I eventually, 2 days before the wedding, decided to wear.
My something borrowed was this marcasite necklace that my Mum got for £5 from a car boot sale. I'm not sure if I've broken the 'something borrowed' rules but my Mum has now given me the necklace to keep (yay). I was bought a marcasite watch for my 16th birthday so I wore that too. Unfortunately I noticed too late that the battery had died - so I have no idea if I actually got to the ceremony on time. I hate not knowing what time it is so after the meal I changed to a working watch.