Most travel insurance companies valuables/baggage clause are on an "Indemnity" basis; this means that the insurer will place a value against the item you are claiming for and make a deduction for wear and tear.
You can query how they got to there original valuation and maybe get a alternative valuation from another jeweller; aletrnativly if you have a Home Contents policy you may find that it has a more favourable clause or claims limit for items away from the home
Re your point about sending the other earring, you can offer to send a photograph of the item, I would question though exactly what the insurers are going to do with one earring??
If you can't reach a solution with the insurer, or you think they are being too demanding about the remaining earring, you final course of action for complaint would be the FInancial Ombudsmans Service (FOS)
TIG