Or you can go to the craft vendor Felicia Worthington in Marleybone outside of Digmore Station. On the last page of her recipes are transmuting recipes. There is one for diamonds. It cost 200 gold. You will need 15 ore to make one diamond. You can buty bundles of ore in the crown shop for 499. it is 100 pieces of ore.
if you grow King Parsley you will get plenty of diamonds over time. Depends on how fast you want to get them. I have over 100 diamonds at the moment just from growing King Parsley regularly. The best thing is that you only need to get one King Parsley seed and in a couple of weeks you can have dozens, as compared to some plants where you can only multiply your supply during double points events.