Theo Fennel Cross Pendant

by Caroline
(Lincoln UK)

Hi, somebody bought me a Theo Fennell necklace about 6/7 years ago - it has a small cross (about 10mm) I am not sure if it is aquamarine (possibility it could be Topaz) on an 18 carat white gold chain - the hallmarkings are 750 and I think TT opposite, and it looks like a 4 or an A on the other side. Any idea what this means and if it is of any value? Many thanks