Praise be to Allaah.
a) You should know the present market value – theoretically something like taking the gold to market and ask the goldsmith:”Look I want to sell this, how much you are going to pay me?” So, he will offer a price depending on the type and age of your gold. You should pay zakaah on that!!
Please refer to the following ruling from Islam-Q-A:
b) If you are not trading stones and diamonds – they are just your personal assets, then you do not have to pay zakaah on those.
Again, please refer to the following ruling from Islam-Q-A:
And Allah knows the best!