Question
 

I cannot seem to remember the name of the gem/stone that you can buy and use at vendors to sell and buy that is similar in price that you can use to convert your copper/silver over to plat. Anyone out there know what I am talking about?

Not sure which gem you are referring to, but all of the gems (except that wannabe gem the Lambent Stone) have a very similar buy/sell price, which makes them excellent for converting lesser coins.

I was thinking there was a certain gem that had the best exchange rate.

I always did opals or jade

Not sure if there are differing conversions but monks on Cazic Thule used peridots due to their near-10p sale. Made for nice even numbers in purchases.

My monk always kept a stack of perodots and a stack of rubys. It was so convenient that I do it on all my chars now.

Exchange rates for standard vendor sold gems don't vary at all that I ever noticed, live or here. Just gotta get you CHA high enough to get best price (not very high, 110-ish? sombody worked out the exact number.) Peridots are convenient for being 10p base value and I've never met the cleric who'd refuse 'dots as an honorarium for a rez.

I know Jasper for sure is a 1:1 trade for sure as when i need abit of plat on another char and need to do a drop transfer they work well for me. Think that a couple others are similar but some you will lose abit of plat.

I've heard carnelian is great; pretty sure its sold for 1 plat, sells back for 9g?

Really they're right, it's not that it's a better exchange rate it's just that the prive is so relatively low that the 10% loss seems like much less of a disparity than say 10% of the cost of a gem that might cost 100+plat, though if you were t sell 100 plat worth of carnelians you've lose the same mount just in smaller doses.