since each gem is tied to a particular mining spot and some people never go back to older spots why don't we all just do an 'I have blank to spare I'd like blank' like someone has a crud ton of amethyst (me) but wants emeralds and well due to pickaxe durability and attempts to level up mining can't go and spend a few weeks mining for emeralds they can offer amethyst for emeralds at a one to one (or any agreed upon rate)
edit seems all the gems sell the same for the vendor
as far as I know it goes citrine first stop emerald the second third amethyst, fourth can someone else fill in the blanks here?